repo_zf1/documentation/manual/en/module_specs/Zend_Feed_Reader.xml

<?xml version="1.0" encoding="UTF-8"?>
<!-- Reviewed: no -->
<sect1 id="zend.feed.reader">
    <title>Zend_Feed_Reader</title>

    <sect2 id="zend.feed.reader.introduction">
        <title>Introduction</title>

        <para>
            <classname>Zend_Feed_Reader</classname> is a component used to
            consume <acronym>RSS</acronym> and Atom feeds of any version, including
            <acronym>RDF</acronym>/<acronym>RSS</acronym> 1.0,
            <acronym>RSS</acronym> 2.0 and Atom 0.3/1.0. The <acronym>API</acronym> for
            retrieving feed data is
            deliberately simple since <classname>Zend_Feed_Reader</classname> is
            capable of searching any feed of any type for the information
            requested through the <acronym>API</acronym>. If the typical elements containing this
            information are not present, it will adapt and fall back on a
            variety of alternative elements instead. This ability to choose from
            alternatives removes the need for users to create their own
            abstraction layer on top of the component to make it useful or have
            any in-depth knowledge of the underlying standards, current
            alternatives, and namespaced extensions.
        </para>

        <para>
            Internally, <classname>Zend_Feed_Reader</classname> works almost
            entirely on the basis of making XPath queries against the feed <acronym>XML</acronym>'s
            Document Object Model. The <acronym>DOM</acronym> is not exposed though a chained
            property <acronym>API</acronym> like <classname>Zend_Feed</classname> though the
            underlying <classname>DOMDocument</classname>,
            <classname>DOMElement</classname> and
            <classname>DOMXPath</classname> objects are exposed for external
            manipulation. This singular approach to parsing is consistent and
            the component offers a plugin system to add to the Feed and Entry
            level <acronym>API</acronym> by writing Extensions on a similar basis.
        </para>

        <para>
            Performance is assisted in three ways. First of all,
            <classname>Zend_Feed_Reader</classname> supports caching using
            <classname>Zend_Cache</classname> to maintain a copy of the original
            feed <acronym>XML</acronym>. This allows you to skip network requests for a feed
            <acronym>URI</acronym> if
            the cache is valid. Second, the Feed and Entry level <acronym>API</acronym> is backed
            by an internal cache (non-persistant) so repeat <acronym>API</acronym> calls for the
            same feed will avoid additional <acronym>DOM</acronym>/XPath use. Thirdly, importing
            feeds from a <acronym>URI</acronym> can take advantage of
            <acronym>HTTP</acronym> Conditional GET requests
            which allow servers to issue an empty 304 response when the
            requested feed has not changed since the last time you requested it.
            In the final case, an instance of <classname>Zend_Cache</classname>
            will hold the last received feed along with the ETag and
            Last-Modified header values sent in the <acronym>HTTP</acronym> response.
        </para>

        <para>
            In relation to <classname>Zend_Feed</classname>,
            <classname>Zend_Feed_Reader</classname> was formulated as a free
            standing replacement for <classname>Zend_Feed</classname> but it is
            not backwards compatible with <classname>Zend_Feed</classname>.
            Rather it is an alternative following a different ideology focused
            on being simple to use, flexible, consistent and extendable through
            the plugin system. <classname>Zend_Feed_Reader</classname> is also
            not capable of constructing feeds and delegates this responsibility
            to <classname>Zend_Feed_Writer</classname>, its sibling in arms.
        </para>
    </sect2>

    <sect2 id="zend.feed.reader.import">
        <title>Importing Feeds</title>

        <para>
            Importing a feed with <classname>Zend_Feed_Reader</classname> is not
            that much different to <classname>Zend_Feed</classname>. Feeds can
            be imported from a string, file, <acronym>URI</acronym> or an instance of type
            <classname>Zend_Feed_Abstract</classname>. Importing from a <acronym>URI</acronym> can
            additionally utilise a <acronym>HTTP</acronym> Conditional GET request. If importing
            fails, an exception will be raised. The end result will be an object
            of type <classname>Zend_Feed_Reader_FeedInterface</classname>, the
            core implementations of which are
            <classname>Zend_Feed_Reader_Feed_Rss</classname> and
            <classname>Zend_Feed_Reader_Feed_Atom</classname>
            (<classname>Zend_Feed</classname> took all the short names!). Both
            objects support multiple (all existing) versions of these broad feed
            types.
        </para>

        <para>
            In the following example, we import an <acronym>RDF</acronym>/<acronym>RSS</acronym> 1.0
            feed and extract some basic information that can be saved to a database or
            elsewhere.
        </para>

        <programlisting language="php"><![CDATA[
$feed = Zend_Feed_Reader::import('http://www.planet-php.net/rdf/');
$data = array(
    'title'        => $feed->getTitle(),
    'link'         => $feed->getLink(),
    'dateModified' => $feed->getDateModified(),
    'description'  => $feed->getDescription(),
    'language'     => $feed->getLanguage(),
    'entries'      => array(),
);

foreach ($feed as $entry) {
    $edata = array(
        'title'        => $entry->getTitle(),
        'description'  => $entry->getDescription(),
        'dateModified' => $entry->getDateModified(),
        'authors'       => $entry->getAuthors(),
        'link'         => $entry->getLink(),
        'content'      => $entry->getContent()
    );
    $data['entries'][] = $edata;
}
]]></programlisting>

        <para>
            The example above demonstrates
            <classname>Zend_Feed_Reader</classname>'s <acronym>API</acronym>, and it also
            demonstrates some of its internal operation. In reality, the <acronym>RDF</acronym>
            feed selected does not have any native date or author elements,
            however it does utilise the Dublin Core 1.1 module which offers
            namespaced creator and date elements.
            <classname>Zend_Feed_Reader</classname> falls back on these and
            similar options if no relevant native elements exist. If it
            absolutely cannot find an alternative it will return <constant>NULL</constant>,
            indicating the information could not be found in the feed. You
            should note that classes implementing
            <classname>Zend_Feed_Reader_FeedInterface</classname> also implement
            the <acronym>SPL</acronym> <classname>Iterator</classname> and
            <classname>Countable</classname> interfaces.
        </para>

        <para>
            Feeds can also be imported from strings, files, and even objects of
            type <classname>Zend_Feed_Abstract</classname>.
        </para>

        <programlisting language="php"><![CDATA[
// from a URI
$feed = Zend_Feed_Reader::import('http://www.planet-php.net/rdf/');

// from a String
$feed = Zend_Feed_Reader::importString($feedXmlString);

// from a file
$feed = Zend_Feed_Reader::importFile('./feed.xml');

// from a Zend_Feed_Abstract object
$zfeed = Zend_Feed::import('http://www.planet-php.net/atom/');
$feed  = Zend_Feed_Reader::importFeed($zfeed);
]]></programlisting>
    </sect2>

    <sect2 id="zend.feed.reader.sources">
        <title>Retrieving Underlying Feed and Entry Sources</title>

        <para>
            <classname>Zend_Feed_Reader</classname> does its best not to stick
            you in a narrow confine. If you need to work on a feed outside of
            <classname>Zend_Feed_Reader</classname>, you can extract the base
            <classname>DOMDocument</classname> or
            <classname>DOMElement</classname> objects from any class, or even an
            <acronym>XML</acronym> string containing these. Also provided are methods to extract
            the current <classname>DOMXPath</classname> object (with all core
            and Extension namespaces registered) and the correct prefix used in
            all XPath queries for the current Feed or Entry. The basic methods
            to use (on any object) are <methodname>saveXml()</methodname>,
            <methodname>getDomDocument()</methodname>,
            <methodname>getElement()</methodname>,
            <methodname>getXpath()</methodname> and
            <methodname>getXpathPrefix()</methodname>. These will let you break
            free of <classname>Zend_Feed_Reader</classname> and do whatever else
            you want.
        </para>

        <itemizedlist>
            <listitem>
                <para>
                    <methodname>saveXml()</methodname> returns an <acronym>XML</acronym> string
                    containing only the element representing the current object.
                </para>
            </listitem>

            <listitem>
                <para>
                    <methodname>getDomDocument()</methodname> returns the
                    <classname>DOMDocument</classname> object representing the
                    entire feed (even if called from an Entry object).
                </para>
            </listitem>

            <listitem>
                <para>
                    <methodname>getElement()</methodname> returns the
                    <classname>DOMElement</classname> of the current object
                    (i.e. the Feed or current Entry).
                </para>
            </listitem>

            <listitem>
                <para>
                    <methodname>getXpath()</methodname> returns the
                    <classname>DOMXPath</classname> object for the current feed
                    (even if called from an Entry object) with the namespaces of
                    the current feed type and all loaded Extensions
                    pre-registered.
                </para>
            </listitem>

            <listitem>
                <para>
                    <methodname>getXpathPrefix()</methodname> returns the query
                    prefix for the current object (i.e. the Feed or current
                    Entry) which includes the correct XPath query path for that
                    specific Feed or Entry.
                </para>
            </listitem>
        </itemizedlist>

        <para>
            Here's an example where a feed might include an <acronym>RSS</acronym> Extension not
            supported by <classname>Zend_Feed_Reader</classname> out of the box.
            Notably, you could write and register an Extension (covered later)
            to do this, but that's not always warranted for a quick check. You
            must register any new namespaces on the
            <classname>DOMXPath</classname> object before use unless they are
            registered by <classname>Zend_Feed_Reader</classname> or an
            Extension beforehand.
        </para>

        <programlisting language="php"><![CDATA[
$feed        = Zend_Feed_Reader::import('http://www.planet-php.net/rdf/');
$xpathPrefix = $feed->getXpathPrefix();
$xpath       = $feed->getXpath();
$xpath->registerNamespace('admin', 'http://webns.net/mvcb/');
$reportErrorsTo = $xpath->evaluate('string('
                                 . $xpathPrefix
                                 . '/admin:errorReportsTo)');
]]></programlisting>

        <warning>
            <para>
                If you register an already registered namespace with a different
                prefix name to that used internally by
                <classname>Zend_Feed_Reader</classname>, it will break the
                internal operation of this component.
            </para>
        </warning>
    </sect2>

    <sect2 id="zend.feed.reader.cache-request">
        <title>Cache Support and Intelligent Requests</title>

        <sect3 id="zend.feed.reader.cache-request.cache">
            <title>Adding Cache Support to Zend_Feed_Reader</title>

            <para>
                <classname>Zend_Feed_Reader</classname> supports using an
                instance of <classname>Zend_Cache</classname> to cache feeds (as
                <acronym>XML</acronym>) to avoid unnecessary network requests. Adding a cache is as
                simple here as it is for other Zend Framework components, create
                and configure your cache and then tell
                <classname>Zend_Feed_Reader</classname> to use it! The cache key
                used is "<classname>Zend_Feed_Reader_</classname>" followed by the
                <acronym>MD5</acronym> hash of the feed's <acronym>URI</acronym>.
            </para>

            <programlisting language="php"><![CDATA[
$frontendOptions = array(
   'lifetime' => 7200,
   'automatic_serialization' => true
);
$backendOptions = array('cache_dir' => './tmp/');
$cache = Zend_Cache::factory(
    'Core', 'File', $frontendOptions, $backendOptions
);

Zend_Feed_Reader::setCache($cache);
]]></programlisting>

            <note>
                <para>
                    While it's a little off track, you should also consider
                    adding a cache to
                    <classname>Zend_Loader_PluginLoader</classname> which is
                    used by <classname>Zend_Feed_Reader</classname> to load
                    Extensions.
                </para>
            </note>
        </sect3>

        <sect3 id="zend.feed.reader.cache-request.http-conditional-get">
            <title>HTTP Conditional GET Support</title>

            <para>
                The big question often asked when importing a feed frequently, is
                if it has even changed. With a cache enabled, you can add <acronym>HTTP</acronym>
                Conditional GET support to your arsenal to answer that question.
            </para>

            <para>
                Using this method, you can request feeds from <acronym>URI</acronym>s and include
                their last known ETag and Last-Modified response header values
                with the request (using the If-None-Match and If-Modified-Since
                headers). If the feed on the server remains unchanged, you
                should receive a 304 response which tells
                <classname>Zend_Feed_Reader</classname> to use the cached
                version. If a full feed is sent in a response with a status code
                of 200, this means the feed has changed and
                <classname>Zend_Feed_Reader</classname> will parse the new
                version and save it to the cache. It will also cache the new
                ETag and Last-Modified header values for future use.
            </para>

            <para>
                These "conditional" requests are not guaranteed to be supported
                by the server you request a <acronym>URI</acronym> of, but can be attempted
                regardless. Most common feed sources like blogs should however
                have this supported. To enable conditional requests, you will
                need to provide a cache to <classname>Zend_Feed_Reader</classname>.
            </para>

            <programlisting language="php"><![CDATA[
$frontendOptions = array(
   'lifetime' => 86400,
   'automatic_serialization' => true
);
$backendOptions = array('cache_dir' => './tmp/');
$cache = Zend_Cache::factory(
    'Core', 'File', $frontendOptions, $backendOptions
);

Zend_Feed_Reader::setCache($cache);
Zend_Feed_Reader::useHttpConditionalGet();

$feed = Zend_Feed_Reader::import('http://www.planet-php.net/rdf/');
]]></programlisting>

            <para>
                In the example above, with <acronym>HTTP</acronym> Conditional GET requests enabled,
                the response header values for ETag and Last-Modified will be cached
                along with the feed. For the next 24hrs (the cache lifetime), feeds will
                only be updated on the cache if a non-304 response is received
                containing a valid <acronym>RSS</acronym> or Atom <acronym>XML</acronym> document.
            </para>

            <para>
                If you intend on managing request headers from outside
                <classname>Zend_Feed_Reader</classname>, you can set the
                relevant If-None-Matches and If-Modified-Since request headers
                via the <acronym>URI</acronym> import method.
            </para>

            <programlisting language="php"><![CDATA[
$lastEtagReceived = '5e6cefe7df5a7e95c8b1ba1a2ccaff3d';
$lastModifiedDateReceived = 'Wed, 08 Jul 2009 13:37:22 GMT';
$feed = Zend_Feed_Reader::import(
    $uri, $lastEtagReceived, $lastModifiedDateReceived
);
]]></programlisting>
        </sect3>
    </sect2>

    <sect2 id="zend.feed.reader.locate">
        <title>Locating Feed URIs from Websites</title>

        <para>
            These days, many websites are aware that the location of their <acronym>XML</acronym>
            feeds is not always obvious. A small <acronym>RDF</acronym>, <acronym>RSS</acronym> or
            Atom graphic helps when the user is reading the page, but what about when a machine
            visits trying to identify where your feeds are located? To assist in
            this, websites may point to their feeds using &lt;link&gt; tags in
            the &lt;head&gt; section of their <acronym>HTML</acronym>. To take advantage of this,
            you can use <classname>Zend_Feed_Reader</classname> to locate these
            feeds using the static <methodname>findFeedLinks()</methodname>
            method.
        </para>

        <para>
            This method calls any <acronym>URI</acronym> and searches for the location of
            <acronym>RSS</acronym>, <acronym>RDF</acronym>
            and Atom feeds assuming the website's <acronym>HTML</acronym> contains the relevant
            links. It then returns a value object where you can check for the existence of a
            <acronym>RSS</acronym>, <acronym>RDF</acronym> or Atom feed <acronym>URI</acronym>.
        </para>

        <para>
            The returned object is an <classname>ArrayObject</classname> subclass
            called <classname>Zend_Feed_Reader_Collection_FeedLink</classname> so you can cast
            it to an array, or iterate over it, to access all the detected links.
            However, as a simple shortcut, you can just grab the first RSS, RDF
            or Atom link using its public properties as in the example below. Otherwise,
            each element of the <classname>ArrayObject</classname> is a simple array
            with the keys "type" and "uri" where the type is one of "rdf", "rss" or
            "atom".
        </para>

        <programlisting language="php"><![CDATA[
$links = Zend_Feed_Reader::findFeedLinks('http://www.planet-php.net');

if(isset($links->rdf)) {
    echo $links->rdf, "\n"; // http://www.planet-php.org/rdf/
}
if(isset($links->rss)) {
    echo $links->rss, "\n"; // http://www.planet-php.org/rss/
}
if(isset($links->atom)) {
    echo $links->atom, "\n"; // http://www.planet-php.org/atom/
}
]]></programlisting>

        <para>
            Based on these links, you can then import from whichever source you
            wish in the usual manner.
        </para>

        <para>
            This quick method only gives you one link for each feed type, but
            websites may indicate many links of any type. Perhaps it's a news
            site with a RSS feed for each news category. You can iterate over
            all links using the ArrayObject's iterator.
        </para>

        <programlisting language="php"><![CDATA[
$links = Zend_Feed_Reader::findFeedLinks('http://www.planet-php.net');

foreach ($links as $link) {
    echo $link['uri'], "\n";
}
]]></programlisting>
  </sect2>

  <sect2 id="zend.feed.reader.attribute-collections">
        <title>Attribute Collections</title>

        <para>
            In an attempt to simplify return types, with Zend Framework 1.10 return
            types from the various feed and entry level methods may include an object
            of type <classname>Zend_Feed_Reader_Collection_CollectionAbstract</classname>.
            Despite the special class name which I'll explain below, this is just a simple
            subclass of SPL's <classname>ArrayObject</classname>.
        </para>

        <para>
            The main purpose here is to allow the presentation of as much data as possible
            from the requested elements, while still allowing access to the most relevant
            data as a simple array. This also enforces a standard approach to returning
            such data which previously may have wandered between arrays and objects.
        </para>

        <para>
            The new class type acts identically to <classname>ArrayObject</classname>
            with the sole addition being a new method <methodname>getValues()</methodname>
            which returns a simple flat array containing the most relevant information.
        </para>

        <para>
            A simple example of this is
            <methodname>Zend_Feed_Reader_FeedInterface::getCategories()</methodname>. When used with
            any RSS or Atom feed, this method will return category data as a container object called
            <classname>Zend_Feed_Reader_Collection_Category</classname>. The container object will
            contain, per category, three fields of data: term, scheme and label. The "term" is the
            basic category name, often machine readable (i.e. plays nice with URIs). The scheme
            represents a categorisation scheme (usually a URI identifier) also known as a "domain"
            in RSS 2.0. The "label" is a human readable category name which supports html entities.
            In RSS 2.0, there is no label attribute so it is always set to the same value as the
            term for convenience.
        </para>

        <para>
            To access category labels by themselves in a simple value array,
            you might commit to something like:
        </para>

        <programlisting language="php"><![CDATA[
$feed = Zend_Feed_Reader::import('http://www.example.com/atom.xml');
$categories = $feed->getCategories();
$labels = array();
foreach ($categories as $cat) {
    $labels[] = $cat['label']
}
]]></programlisting>

        <para>
            It's a contrived example, but the point is that the labels are tied up with
            other information.
        </para>

        <para>
            However, the container class allows you to access the "most relevant" data
            as a simple array using the <methodname>getValues()</methodname> method. The concept
            of "most relevant" is obviously a judgement call. For categories it means the category
            labels (not the terms or schemes) while for authors it would be the authors' names
            (not their email addresses or URIs). The simple array is flat (just values) and passed
            through <methodname>array_unique()</methodname> to remove duplication.
        </para>

        <programlisting language="php"><![CDATA[
$feed = Zend_Feed_Reader::import('http://www.example.com/atom.xml');
$categories = $feed->getCategories();
$labels = $categories->getValues();
]]></programlisting>

        <para>
            The above example shows how to extract only labels and nothing else thus
            giving simple access to the category labels without any additional work to extract
            that data by itself.
        </para>
  </sect2>

  <sect2 id="zend.feed.reader.retrieve-info">
        <title>Retrieving Feed Information</title>

        <para>
            Retrieving information from a feed (we'll cover entries/items in the
            next section though they follow identical principals) uses a clearly
            defined <acronym>API</acronym> which is exactly the same regardless of whether the feed
            in question is <acronym>RSS</acronym>/<acronym>RDF</acronym>/Atom. The same goes for
            sub-versions of these standards and we've tested every single
            <acronym>RSS</acronym> and Atom version. While
            the underlying feed <acronym>XML</acronym> can differ substantially in terms of the
            tags and elements they present, they nonetheless are all trying to
            convey similar information and to reflect this all the differences
            and wrangling over alternative tags are handled internally by
            <classname>Zend_Feed_Reader</classname> presenting you with an
            identical interface for each. Ideally, you should not have to care
            whether a feed is <acronym>RSS</acronym> or Atom so long as you can extract the
            information you want.
        </para>

        <note>
            <para>
                While determining common ground between feed types is itself complex, it
                should be noted that RSS in particular is a constantly disputed "specification".
                This has its roots in the original RSS 2.0 document which contains ambiguities
                and does not detail the correct treatment of all elements. As a result, this
                component rigorously applies the RSS 2.0.11 Specification published by the
                RSS Advisory Board and its accompanying RSS Best Practices Profile. No
                other interpretation of RSS 2.0 will be supported though exceptions may
                be allowed where it does not directly prevent the application of the two
                documents mentioned above.
            </para>
        </note>

        <para>
            Of course, we don't live in an ideal world so there may be times the
            <acronym>API</acronym> just does not cover what you're looking for. To assist you,
            <classname>Zend_Feed_Reader</classname> offers a plugin system which
            allows you to write Extensions to expand the core <acronym>API</acronym> and cover any
            additional data you are trying to extract from feeds. If writing
            another Extension is too much trouble, you can simply grab the
            underlying <acronym>DOM</acronym> or XPath objects and do it by hand in your
            application. Of course, we really do encourage writing an Extension
            simply to make it more portable and reusable, and useful Extensions may be proposed
            to the Framework for formal addition.
        </para>

        <para>
            Here's a summary of the Core <acronym>API</acronym> for Feeds. You should note it
            comprises not only the basic <acronym>RSS</acronym> and Atom standards, but also
            accounts for a number of included Extensions bundled with
            <classname>Zend_Feed_Reader</classname>. The naming of these
            Extension sourced methods remain fairly generic - all Extension
            methods operate at the same level as the Core <acronym>API</acronym> though we do allow
            you to retrieve any specific Extension object separately if required.
        </para>

        <table>
            <title>Feed Level API Methods</title>

            <tgroup cols="2">
                <tbody>
                    <row>
                        <entry><methodname>getId()</methodname></entry>
                        <entry>Returns a unique ID associated with this feed</entry>
                    </row>

                    <row>
                        <entry><methodname>getTitle()</methodname></entry>
                        <entry>Returns the title of the feed</entry>
                    </row>

                    <row>
                        <entry><methodname>getDescription()</methodname></entry>
                        <entry>Returns the text description of the feed.</entry>
                    </row>

                    <row>
                        <entry><methodname>getLink()</methodname></entry>

                        <entry>
                            Returns a <acronym>URI</acronym> to the <acronym>HTML</acronym> website
                            containing the same or
                            similar information as this feed (i.e. if the feed is from a blog,
                            it should provide the blog's <acronym>URI</acronym> where the
                            <acronym>HTML</acronym> version of the entries can be read).
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getFeedLink()</methodname></entry>

                        <entry>
                            Returns the <acronym>URI</acronym> of this feed, which may be the
                            same as the <acronym>URI</acronym> used to import the feed. There
                            are important cases where the feed link may differ because the source
                            URI is being updated and is intended to be removed in the future.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getAuthors()</methodname></entry>

                        <entry>
                            Returns an object of type
                            <classname>Zend_Feed_Reader_Collection_Author</classname> which is an
                            <classname>ArrayObject</classname> whose elements are each simple arrays
                            containing any combination of the keys "name", "email" and "uri". Where
                            irrelevant to the source data, some of these keys may be omitted.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getAuthor(integer $index = 0)</methodname></entry>

                        <entry>
                            Returns either the first author known, or with the
                            optional <varname>$index</varname> parameter any specific
                            index on the array of Authors as described above (returning
                            <constant>NULL</constant> if an invalid index).
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getDateCreated()</methodname></entry>

                        <entry>
                            Returns the date on which this feed was created. Generally
                            only applicable to Atom where it represents the date the resource
                            described by an Atom 1.0 document was created. The returned date
                            will be a <classname>Zend_Date</classname> object.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getDateModified()</methodname></entry>

                        <entry>
                            Returns the date on which this feed was last modified. The returned date
                            will be a <classname>Zend_Date</classname> object.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getLanguage()</methodname></entry>

                        <entry>
                            Returns the language of the feed (if defined) or simply the
                            language noted in the <acronym>XML</acronym> document.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getGenerator()</methodname></entry>

                        <entry>
                            Returns the generator of the feed, e.g. the software which
                            generated it. This may differ between <acronym>RSS</acronym> and Atom
                            since Atom defines a different notation.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getCopyright()</methodname></entry>
                        <entry>Returns any copyright notice associated with the feed.</entry>
                    </row>

                    <row>
                        <entry><methodname>getHubs()</methodname></entry>

                        <entry>
                            Returns an array of all Hub Server <acronym>URI</acronym> endpoints
                            which are advertised by the feed for use with the Pubsubhubbub
                            Protocol, allowing subscriptions to the feed for real-time updates.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getCategories()</methodname></entry>

                        <entry>
                            Returns a <classname>Zend_Feed_Reader_Collection_Category</classname>
                            object containing the details of any categories associated with the
                            overall feed. The supported fields include "term" (the machine readable
                            category name), "scheme" (the categorisation scheme/domain for this
                            category), and "label" (a html decoded human readable category name).
                            Where any of the three fields are absent from the field, they are either
                            set to the closest available alternative or, in the case of "scheme",
                            set to <constant>NULL</constant>.
                        </entry>
                    </row>
                </tbody>
            </tgroup>
        </table>

        <para>
            Given the variety of feeds in the wild, some of these methods will
            undoubtedly return <constant>NULL</constant> indicating the relevant information
            couldn't be located. Where possible, <classname>Zend_Feed_Reader</classname>
            will fall back on alternative elements during its search. For
            example, searching an <acronym>RSS</acronym> feed for a modification date is more
            complicated than it looks. <acronym>RSS</acronym> 2.0 feeds should include a
            <command>&lt;lastBuildDate&gt;</command> tag and (or) a
            <command>&lt;pubDate&gt;</command> element. But what if it doesn't, maybe
            this is an <acronym>RSS</acronym> 1.0 feed? Perhaps it instead has an
            <command>&lt;atom:updated&gt;</command> element with identical information
            (Atom may be used to supplement <acronym>RSS</acronym>'s syntax)? Failing that, we
            could simply look at the entries, pick the most recent, and use its
            <command>&lt;pubDate&gt;</command> element. Assuming it exists... Many
            feeds also use Dublin Core 1.0/1.1 <command>&lt;dc:date&gt;</command>
            elements for feeds and entries. Or we could find Atom lurking again.
        </para>

        <para>
            The point is, <classname>Zend_Feed_Reader</classname> was designed
            to know this. When you ask for the modification date (or anything
            else), it will run off and search for all these alternatives until
            it either gives up and returns <constant>NULL</constant>, or finds an
            alternative that should have the right answer.
        </para>

        <para>
            In addition to the above methods, all Feed objects implement methods
            for retrieving the <acronym>DOM</acronym> and XPath objects for the current feeds as
            described earlier. Feed objects also implement the <acronym>SPL</acronym> Iterator and
            Countable interfaces. The extended <acronym>API</acronym> is summarised below.
        </para>

        <table>
            <title>Extended Feed Level API Methods</title>

            <tgroup cols="2">
                <tbody>
                    <row>
                        <entry><methodname>getDomDocument()</methodname></entry>

                        <entry>
                            Returns the parent
                            <classname>DOMDocument</classname> object for the
                            entire source <acronym>XML</acronym> document
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getElement()</methodname></entry>

                        <entry>
                            Returns the current feed level
                            <classname>DOMElement</classname> object
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>saveXml()</methodname></entry>

                        <entry>
                            Returns a string containing an <acronym>XML</acronym> document of the
                            entire feed element (this is not the original
                            document but a rebuilt version)
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getXpath()</methodname></entry>

                        <entry>
                            Returns the <classname>DOMXPath</classname> object
                            used internally to run queries on the
                            <classname>DOMDocument</classname> object (this
                            includes core and Extension namespaces
                            pre-registered)
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getXpathPrefix()</methodname></entry>

                        <entry>
                            Returns the valid <acronym>DOM</acronym> path prefix prepended
                            to all XPath queries matching the feed being queried
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getEncoding()</methodname></entry>

                        <entry>
                            Returns the encoding of the source <acronym>XML</acronym> document
                            (note: this cannot account for errors such as the
                            server sending documents in a different encoding). Where not
                            defined, the default UTF-8 encoding of Unicode is applied.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>count()</methodname></entry>

                        <entry>
                            Returns a count of the entries or items this feed contains
                            (implements <acronym>SPL</acronym> <classname>Countable</classname>
                            interface)
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>current()</methodname></entry>

                        <entry>
                            Returns either the current entry (using the current index
                            from <methodname>key()</methodname>)
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>key()</methodname></entry>
                        <entry>Returns the current entry index</entry>
                    </row>

                    <row>
                        <entry><methodname>next()</methodname></entry>
                        <entry>Increments the entry index value by one</entry>
                    </row>

                    <row>
                        <entry><methodname>rewind()</methodname></entry>
                        <entry>Resets the entry index to 0</entry>
                    </row>

                    <row>
                        <entry><methodname>valid()</methodname></entry>

                        <entry>
                            Checks that the current entry index is valid, i.e.
                            it does fall below 0 and does not exceed the number
                            of entries existing.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getExtensions()</methodname></entry>

                        <entry>
                            Returns an array of all Extension objects loaded for
                            the current feed (note: both feed-level and entry-level Extensions
                            exist, and only feed-level Extensions are returned here).
                            The array keys are of the form {ExtensionName}_Feed.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getExtension(string $name)</methodname></entry>

                        <entry>
                            Returns an Extension object for the feed registered under the
                            provided name. This allows more fine-grained access to
                            Extensions which may otherwise be hidden within the implementation
                            of the standard <acronym>API</acronym> methods.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getType()</methodname></entry>

                        <entry>
                            Returns a static class constant (e.g.
                            <constant>Zend_Feed_Reader::TYPE_ATOM_03</constant>,
                            i.e. Atom 0.3) indicating exactly what kind of feed
                            is being consumed.
                        </entry>
                    </row>
                </tbody>
            </tgroup>
        </table>
    </sect2>

    <sect2 id="zend.feed.reader.entry">
        <title>Retrieving Entry/Item Information</title>

        <para>
            Retrieving information for specific entries or items (depending on
            whether you speak Atom or <acronym>RSS</acronym>) is identical to feed level data.
            Accessing entries is simply a matter of iterating over a Feed object
            or using the <acronym>SPL</acronym> <classname>Iterator</classname> interface Feed
            objects implement and calling the appropriate method on each.
        </para>

        <table>
            <title>Entry Level API Methods</title>

            <tgroup cols="2">
                <tbody>
                    <row>
                        <entry><methodname>getId()</methodname></entry>
                        <entry>Returns a unique ID for the current entry.</entry>
                    </row>

                    <row>
                        <entry><methodname>getTitle()</methodname></entry>
                        <entry>Returns the title of the current entry.</entry>
                    </row>

                    <row>
                        <entry><methodname>getDescription()</methodname></entry>
                        <entry>Returns a description of the current entry.</entry>
                    </row>

                    <row>
                        <entry><methodname>getLink()</methodname></entry>

                        <entry>
                            Returns a <acronym>URI</acronym> to the <acronym>HTML</acronym> version
                            of the current entry.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getPermaLink()</methodname></entry>

                        <entry>
                            Returns the permanent link to the current entry. In most cases,
                            this is the same as using <methodname>getLink()</methodname>.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getAuthors()</methodname></entry>

                        <entry>
                            Returns an object of type
                            <classname>Zend_Feed_Reader_Collection_Author</classname> which is an
                            <classname>ArrayObject</classname> whose elements are each simple arrays
                            containing any combination of the keys "name", "email" and "uri". Where
                            irrelevant to the source data, some of these keys may be omitted.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getAuthor(integer $index = 0)</methodname></entry>

                        <entry>
                            Returns either the first author known, or with the
                            optional <varname>$index</varname> parameter any specific
                            index on the array of Authors as described above (returning
                            <constant>NULL</constant> if an invalid index).
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getDateCreated()</methodname></entry>

                        <entry>
                            Returns the date on which the current entry was
                            created. Generally only applicable to Atom where it
                            represents the date the resource described by an
                            Atom 1.0 document was created.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getDateModified()</methodname></entry>

                        <entry>
                            Returns the date on which the current entry was last
                            modified
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getContent()</methodname></entry>

                        <entry>
                            Returns the content of the current entry (this has any
                            entities reversed if possible assuming the content type is
                            <acronym>HTML</acronym>). The description is returned if a
                            separate content element does not exist.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getEnclosure()</methodname></entry>

                        <entry>
                            Returns an array containing the value of all
                            attributes from a multi-media &lt;enclosure&gt; element including
                            as array keys: <emphasis>url</emphasis>,
                            <emphasis>length</emphasis>, <emphasis>type</emphasis>.
                            In accordance with the RSS Best Practices Profile of the RSS
                            Advisory Board, no support is offers for multiple enclosures
                            since such support forms no part of the RSS specification.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getCommentCount()</methodname></entry>

                        <entry>
                            Returns the number of comments made on this entry at the
                            time the feed was last generated
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getCommentLink()</methodname></entry>

                        <entry>
                            Returns a <acronym>URI</acronym> pointing to the <acronym>HTML</acronym>
                            page where comments can be made on this entry
                        </entry>
                    </row>

                    <row>
                        <entry>
                            <methodname>getCommentFeedLink([string $type =
                                'atom'|'rss'])</methodname>
                        </entry>

                        <entry>
                            Returns a <acronym>URI</acronym> pointing to a feed of the provided type
                            containing all comments for this entry (type defaults to
                            Atom/<acronym>RSS</acronym> depending on current feed type).
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getCategories()</methodname></entry>

                        <entry>
                            Returns a <classname>Zend_Feed_Reader_Collection_Category</classname>
                            object containing the details of any categories associated with the
                            entry. The supported fields include "term" (the machine readable
                            category name), "scheme" (the categorisation scheme/domain for this
                            category), and "label" (a html decoded human readable category name).
                            Where any of the three fields are absent from the field, they are either
                            set to the closest available alternative or, in the case of "scheme",
                            set to <constant>NULL</constant>.
                        </entry>
                    </row>
                </tbody>
            </tgroup>
        </table>

        <para>
            The extended <acronym>API</acronym> for entries is identical to that for feeds with the
            exception of the Iterator methods which are not needed here.
        </para>

        <caution>
            <para>
                There is often confusion over the concepts of modified and
                created dates. In Atom, these are two clearly defined concepts
                (so knock yourself out) but in <acronym>RSS</acronym> they are vague.
                <acronym>RSS</acronym> 2.0
                defines a single <emphasis>&lt;pubDate&gt;</emphasis> element
                which typically refers to the date this entry was published,
                i.e. a creation date of sorts. This is not always the case, and
                it may change with updates or not. As a result, if you really
                want to check whether an entry has changed, don't rely on the
                results of <methodname>getDateModified()</methodname>. Instead,
                consider tracking the <acronym>MD5</acronym> hash of three other elements
                concatenated, e.g. using <methodname>getTitle()</methodname>,
                <methodname>getDescription()</methodname> and
                <methodname>getContent()</methodname>. If the entry was truly
                updated, this hash computation will give a different result than
                previously saved hashes for the same entry. This is obviously
                content oriented, and will not assist in detecting changes to other
                relevant elements. Atom feeds should not require such steps.
            </para>

            <para>
                Further muddying the
                waters, dates in feeds may follow different standards. Atom and
                Dublin Core dates should follow <acronym>ISO</acronym> 8601,
                and <acronym>RSS</acronym> dates should
                follow <acronym>RFC</acronym> 822 or <acronym>RFC</acronym> 2822
                which is also common. Date methods
                will throw an exception if <classname>Zend_Date</classname>
                cannot load the date string using one of the above standards, or the
                <acronym>PHP</acronym> recognised possibilities for <acronym>RSS</acronym> dates.
            </para>
        </caution>

        <warning>
            <para>
                The values returned from these methods are not validated. This
                means users must perform validation on all retrieved data
                including the filtering of any <acronym>HTML</acronym> such as from
                <methodname>getContent()</methodname> before it is output from
                your application. Remember that most feeds come from external
                sources, and therefore the default assumption should be that
                they cannot be trusted.
            </para>
        </warning>

        <table>
            <title>Extended Entry Level API Methods</title>

            <tgroup cols="2">
                <tbody>
                    <row>
                        <entry><methodname>getDomDocument()</methodname></entry>

                        <entry>
                            Returns the parent
                            <classname>DOMDocument</classname> object for the
                            entire feed (not just the current entry)
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getElement()</methodname></entry>

                        <entry>
                            Returns the current entry level
                            <classname>DOMElement</classname> object
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getXpath()</methodname></entry>

                        <entry>
                            Returns the <classname>DOMXPath</classname> object
                            used internally to run queries on the
                            <classname>DOMDocument</classname> object (this
                            includes core and Extension namespaces
                            pre-registered)
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getXpathPrefix()</methodname></entry>

                        <entry>
                            Returns the valid <acronym>DOM</acronym> path prefix prepended
                            to all XPath queries matching the entry being queried
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getEncoding()</methodname></entry>

                        <entry>
                            Returns the encoding of the source <acronym>XML</acronym> document
                            (note: this cannot account for errors such as the server sending
                            documents in a different encoding). The default encoding applied
                            in the absence of any other is the UTF-8 encoding of Unicode.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getExtensions()</methodname></entry>

                        <entry>
                            Returns an array of all Extension objects loaded for
                            the current entry (note: both feed-level and entry-level
                            Extensions exist, and only entry-level Extensions are returned
                            here). The array keys are in the form {ExtensionName}_Entry.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getExtension(string $name)</methodname></entry>

                        <entry>
                            Returns an Extension object for the entry registered under the
                            provided name. This allows more fine-grained access to
                            Extensions which may otherwise be hidden within the implementation
                            of the standard <acronym>API</acronym> methods.
                        </entry>
                    </row>

                    <row>
                        <entry><methodname>getType()</methodname></entry>

                        <entry>
                            Returns a static class constant (e.g.
                            <constant>Zend_Feed_Reader::TYPE_ATOM_03</constant>,
                            i.e. Atom 0.3) indicating exactly what kind
                            of feed is being consumed.
                        </entry>
                    </row>
                </tbody>
            </tgroup>
        </table>
    </sect2>

    <sect2 id="zend.feed.reader.extending">
        <title>Extending Feed and Entry APIs</title>

        <para>
            Extending <classname>Zend_Feed_Reader</classname> allows you to add
            methods at both the feed and entry level which cover the retrieval
            of information not already supported by
            <classname>Zend_Feed_Reader</classname>. Given the number of
            <acronym>RSS</acronym> and
            Atom extensions that exist, this is a good thing since
            <classname>Zend_Feed_Reader</classname> couldn't possibly add
            everything.
        </para>

        <para>
            There are two types of Extensions possible, those which retrieve
            information from elements which are immediate children of the root
            element (e.g. <command>&lt;channel&gt;</command> for <acronym>RSS</acronym> or
            <command>&lt;feed&gt;</command> for Atom) and those who retrieve
            information from child elements of an entry (e.g.
            <command>&lt;item&gt;</command> for <acronym>RSS</acronym> or
            <command>&lt;entry&gt;</command> for Atom). On the filesystem these are grouped as
            classes within a namespace based on the extension standard's name. For example,
            internally we have <classname>Zend_Feed_Reader_Extension_DublinCore_Feed</classname>
            and <classname>Zend_Feed_Reader_Extension_DublinCore_Entry</classname>
            classes which are two Extensions implementing Dublin Core
            1.0 and 1.1 support.
        </para>

        <para>
            Extensions are loaded into <classname>Zend_Feed_Reader</classname>
            using <classname>Zend_Loader_PluginLoader</classname>, so their operation
            will be familiar from other Zend Framework components.
            <classname>Zend_Feed_Reader</classname> already bundles a number of
            these Extensions, however those which are not used internally and
            registered by default (so called Core Extensions) must be registered
            to <classname>Zend_Feed_Reader</classname> before they are used. The
            bundled Extensions include:
        </para>

        <table>
            <title>Core Extensions (pre-registered)</title>

            <tgroup cols="2">
                <tbody>
                    <row>
                        <entry>DublinCore (Feed and Entry)</entry>

                        <entry>
                            Implements support for Dublin Core Metadata Element Set 1.0 and 1.1
                        </entry>
                    </row>

                    <row>
                        <entry>Content (Entry only)</entry>
                        <entry>Implements support for Content 1.0</entry>
                    </row>

                    <row>
                        <entry>Atom (Feed and Entry)</entry>
                        <entry>Implements support for Atom 0.3 and Atom 1.0</entry>
                    </row>

                    <row>
                        <entry>Slash</entry>

                        <entry>
                            Implements support for the Slash <acronym>RSS</acronym> 1.0 module
                        </entry>
                    </row>

                    <row>
                        <entry>WellFormedWeb</entry>
                        <entry>Implements support for the Well Formed Web CommentAPI 1.0</entry>
                    </row>

                    <row>
                        <entry>Thread</entry>

                        <entry>
                            Implements support for Atom Threading Extensions as described
                            in <acronym>RFC</acronym> 4685
                        </entry>
                    </row>

                    <row>
                        <entry>Podcast</entry>

                        <entry>
                            Implements support for the Podcast 1.0 <constant>DTD</constant> from
                            Apple
                        </entry>
                    </row>
                </tbody>
            </tgroup>
        </table>

        <para>
            The Core Extensions are somewhat special since they are extremely
            common and multi-faceted. For example, we have a Core Extension for Atom.
            Atom is implemented as an Extension (not just a base class) because it
            doubles as a valid <acronym>RSS</acronym> module - you can insert
            Atom elements into <acronym>RSS</acronym> feeds. I've even seen
            <acronym>RDF</acronym> feeds which use a lot of Atom in place of more
            common Extensions like Dublin Core.
        </para>

        <table>
            <title>Non-Core Extensions (must register manually)</title>

            <tgroup cols="2">
                <tbody>
                    <row>
                        <entry>Syndication</entry>

                        <entry>
                            Implements Syndication 1.0 support for <acronym>RSS</acronym> feeds
                        </entry>
                    </row>

                    <row>
                        <entry>CreativeCommons</entry>

                        <entry>
                            A <acronym>RSS</acronym> module that adds an element at the
                            &lt;channel&gt; or &lt;item&gt; level that specifies which Creative
                            Commons license applies.
                        </entry>
                    </row>
                </tbody>
            </tgroup>
        </table>

        <para>
            The additional non-Core Extensions are offered but not registered to
            <classname>Zend_Feed_Reader</classname> by default. If you want to
            use them, you'll need to tell
            <classname>Zend_Feed_Reader</classname> to load them in advance of
            importing a feed. Additional non-Core Extensions will be included
            in future iterations of the component.
        </para>

        <para>
            Registering an Extension with
            <classname>Zend_Feed_Reader</classname>, so it is loaded and its <acronym>API</acronym>
            is available to Feed and Entry objects, is a simple affair using the
            <classname>Zend_Loader_PluginLoader</classname>. Here we register
            the optional Slash Extension, and discover that it can be directly
            called from the Entry level <acronym>API</acronym> without any effort. Note that
            Extension names are case sensitive and use camel casing for multiple
            terms.
        </para>

        <programlisting language="php"><![CDATA[
Zend_Feed_Reader::registerExtension('Syndication');
$feed = Zend_Feed_Reader::import('http://rss.slashdot.org/Slashdot/slashdot');
$updatePeriod = $feed->current()->getUpdatePeriod();
]]></programlisting>

        <para>
            In the simple example above, we checked how frequently a feed is being updated
            using the <methodname>getUpdatePeriod()</methodname>
            method. Since it's not part of
            <classname>Zend_Feed_Reader</classname>'s core <acronym>API</acronym>, it could only be
            a method supported by the newly registered Syndication Extension.
        </para>

        <para>
            As you can also notice, the new methods from Extensions are accessible from the main
            <acronym>API</acronym> using <acronym>PHP</acronym>'s magic methods. As an alternative,
            you can also directly access any Extension object for a similar result as seen below.
        </para>

        <programlisting language="php"><![CDATA[
Zend_Feed_Reader::registerExtension('Syndication');
$feed = Zend_Feed_Reader::import('http://rss.slashdot.org/Slashdot/slashdot');
$syndication = $feed->getExtension('Syndication');
$updatePeriod = $syndication->getUpdatePeriod();
]]></programlisting>

        <sect3 id="zend.feed.reader.extending.feed">
            <title>Writing Zend_Feed_Reader Extensions</title>

            <para>
                Inevitably, there will be times when the
                <classname>Zend_Feed_Reader</classname> <acronym>API</acronym> is just not capable
                of getting something you need from a feed or entry. You can use
                the underlying source objects, like
                <classname>DOMDocument</classname>, to get these by hand however
                there is a more reusable method available by writing Extensions
                supporting these new queries.
            </para>

            <para>
                As an example, let's take the case of a purely fictitious
                corporation named Jungle Books. Jungle Books have been
                publishing a lot of reviews on books they sell (from external
                sources and customers), which are distributed as an <acronym>RSS</acronym> 2.0
                feed. Their marketing department realises that web applications
                using this feed cannot currently figure out exactly what book is
                being reviewed. To make life easier for everyone, they determine
                that the geek department needs to extend <acronym>RSS</acronym> 2.0 to include a
                new element per entry supplying the <acronym>ISBN</acronym>-10 or
                <acronym>ISBN</acronym>-13 number of
                the publication the entry concerns. They define the new
                <command>&lt;isbn&gt;</command> element quite simply with a standard
                name and namespace <acronym>URI</acronym>:
            </para>

            <programlisting language="php"><![CDATA[
JungleBooks 1.0:
http://example.com/junglebooks/rss/module/1.0/
]]></programlisting>

            <para>
                A snippet of <acronym>RSS</acronym> containing this extension in practice could be
                something similar to:
            </para>

            <programlisting language="php"><![CDATA[
<?xml version="1.0" encoding="utf-8" ?>
<rss version="2.0"
   xmlns:content="http://purl.org/rss/1.0/modules/content/"
   xmlns:jungle="http://example.com/junglebooks/rss/module/1.0/">
<channel>
    <title>Jungle Books Customer Reviews</title>
    <link>http://example.com/junglebooks</link>
    <description>Many book reviews!</description>
    <pubDate>Fri, 26 Jun 2009 19:15:10 GMT</pubDate>
    <jungle:dayPopular>
        http://example.com/junglebooks/book/938
    </jungle:dayPopular>
    <item>
        <title>Review Of Flatland: A Romance of Many Dimensions</title>
        <link>http://example.com/junglebooks/review/987</link>
        <author>Confused Physics Student</author>
        <content:encoded>
        A romantic square?!
        </content:encoded>
        <pubDate>Thu, 25 Jun 2009 20:03:28 -0700</pubDate>
        <jungle:isbn>048627263X</jungle:isbn>
    </item>
</channel>
</rss>
]]></programlisting>

            <para>
                Implementing this new <acronym>ISBN</acronym> element as a simple entry level
                extension would require the following class (using your own class
                namespace outside of Zend).
            </para>

            <programlisting language="php"><![CDATA[
class My_FeedReader_Extension_JungleBooks_Entry
    extends Zend_Feed_Reader_Extension_EntryAbstract
{
    public function getIsbn()
    {
        if (isset($this->_data['isbn'])) {
            return $this->_data['isbn'];
        }
        $isbn = $this->_xpath->evaluate(
            'string(' . $this->getXpathPrefix() . '/jungle:isbn)'
        );
        if (!$isbn) {
            $isbn = null;
        }
        $this->_data['isbn'] = $isbn;
        return $this->_data['isbn'];
    }

    protected function _registerNamespaces()
    {
        $this->_xpath->registerNamespace(
            'jungle', 'http://example.com/junglebooks/rss/module/1.0/'
        );
    }
}
]]></programlisting>

            <para>
                This extension is easy enough to follow. It creates a new method
                <methodname>getIsbn()</methodname> which runs an XPath query on
                the current entry to extract the <acronym>ISBN</acronym> number enclosed by the
                <command>&lt;jungle:isbn&gt;</command> element. It can optionally
                store this to the internal non-persistent cache (no need to keep
                querying the <acronym>DOM</acronym> if it's called again on the same entry). The
                value is returned to the caller. At the end we have a protected
                method (it's abstract so it must exist) which registers the
                Jungle Books namespace for their custom <acronym>RSS</acronym> module. While we
                call this an <acronym>RSS</acronym> module, there's nothing to prevent the same
                element being used in Atom feeds - and all Extensions which use
                the prefix provided by <methodname>getXpathPrefix()</methodname>
                are actually neutral and work on <acronym>RSS</acronym> or Atom feeds with no
                extra code.
            </para>

            <para>
                Since this Extension is stored outside of Zend Framework, you'll
                need to register the path prefix for your Extensions so
                <classname>Zend_Loader_PluginLoader</classname> can find them.
                After that, it's merely a matter of registering the Extension,
                if it's not already loaded, and using it in practice.
            </para>

            <programlisting language="php"><![CDATA[
if(!Zend_Feed_Reader::isRegistered('JungleBooks')) {
    Zend_Feed_Reader::addPrefixPath(
        '/path/to/My/FeedReader/Extension', 'My_FeedReader_Extension'
    );
    Zend_Feed_Reader::registerExtension('JungleBooks');
}
$feed = Zend_Feed_Reader::import('http://example.com/junglebooks/rss');

// ISBN for whatever book the first entry in the feed was concerned with
$firstIsbn = $feed->current()->getIsbn();
]]></programlisting>

            <para>
                Writing a feed level Extension is not much different. The
                example feed from earlier included an unmentioned
                <command>&lt;jungle:dayPopular&gt;</command> element which Jungle
                Books have added to their standard to include a link to the
                day's most popular book (in terms of visitor traffic). Here's
                an Extension which adds a
                <methodname>getDaysPopularBookLink()</methodname> method to the
                feel level <acronym>API</acronym>.
            </para>

            <programlisting language="php"><![CDATA[
class My_FeedReader_Extension_JungleBooks_Feed
    extends Zend_Feed_Reader_Extension_FeedAbstract
{
    public function getDaysPopularBookLink()
    {
        if (isset($this->_data['dayPopular'])) {
            return $this->_data['dayPopular'];
        }
        $dayPopular = $this->_xpath->evaluate(
            'string(' . $this->getXpathPrefix() . '/jungle:dayPopular)'
        );
        if (!$dayPopular) {
            $dayPopular = null;
        }
        $this->_data['dayPopular'] = $dayPopular;
        return $this->_data['dayPopular'];
    }

    protected function _registerNamespaces()
    {
        $this->_xpath->registerNamespace(
            'jungle', 'http://example.com/junglebooks/rss/module/1.0/'
        );
    }
}
]]></programlisting>

            <para>
                Let's repeat the last example using a custom Extension to show the
                method being used.
            </para>

            <programlisting language="php"><![CDATA[
if(!Zend_Feed_Reader::isRegistered('JungleBooks')) {
    Zend_Feed_Reader::addPrefixPath(
        '/path/to/My/FeedReader/Extension', 'My_FeedReader_Extension'
    );
    Zend_Feed_Reader::registerExtension('JungleBooks');
}
$feed = Zend_Feed_Reader::import('http://example.com/junglebooks/rss');

// URI to the information page of the day's most popular book with visitors
$daysPopularBookLink = $feed->getDaysPopularBookLink();

// ISBN for whatever book the first entry in the feed was concerned with
$firstIsbn = $feed->current()->getIsbn();
]]></programlisting>

            <para>
                Going through these examples, you'll note that we don't register
                feed and entry Extensions separately. Extensions within the same
                standard may or may not include both a feed and entry class, so
                <classname>Zend_Feed_Reader</classname> only requires you to
                register the overall parent name, e.g. JungleBooks, DublinCore,
                Slash. Internally, it can check at what level Extensions exist
                and load them up if found. In our case, we have a full set of
                Extensions now: <classname>JungleBooks_Feed</classname> and
                <classname>JungleBooks_Entry</classname>.
            </para>
        </sect3>
    </sect2>
</sect1>