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<sect1 id="zend.validate.writing_validators">
    <title>Writing Validators</title>

    <para>
        <classname>Zend_Validate</classname> supplies a set of commonly needed validators, but
        inevitably, developers will wish to write custom validators for their particular needs. The
        task of writing a custom validator is described in this section.
    </para>

    <para>
        <classname>Zend_Validate_Interface</classname> defines two methods,
        <methodname>isValid()</methodname> and <methodname>getMessages()</methodname>, that may
        be implemented by user classes in order to create custom validation objects. An object that
        implements <classname>Zend_Validate_Interface</classname> interface may be added to a
        validator chain with <methodname>Zend_Validate::addValidator()</methodname>. Such objects
        may also be used with <link
            linkend="zend.filter.input"><classname>Zend_Filter_Input</classname></link>.
    </para>

    <para>
        As you may already have inferred from the above description of
        <classname>Zend_Validate_Interface</classname>, validation classes provided with Zend
        Framework return a boolean value for whether or not a value validates successfully. They
        also provide information about <emphasis>why</emphasis> a value failed validation. The
        availability of the reasons for validation failures may be valuable to an application for
        various purposes, such as providing statistics for usability analysis.
    </para>

    <para>
        Basic validation failure message functionality is implemented in
        <classname>Zend_Validate_Abstract</classname>. To include this functionality when creating a
        validation class, simply extend <classname>Zend_Validate_Abstract</classname>. In the
        extending class you would implement the <methodname>isValid()</methodname> method logic and
        define the message variables and message templates that correspond to the types of
        validation failures that can occur. If a value fails your validation tests, then
        <methodname>isValid()</methodname> should return <constant>FALSE</constant>. If the value
        passes your validation tests, then <methodname>isValid()</methodname> should return
        <constant>TRUE</constant>.
    </para>

    <para>
        In general, the <methodname>isValid()</methodname> method should not throw any exceptions,
        except where it is impossible to determine whether or not the input value is valid. A few
        examples of reasonable cases for throwing an exception might be if a file cannot be opened,
        an <acronym>LDAP</acronym> server could not be contacted, or a database connection is
        unavailable, where such a thing may be required for validation success or failure to be
        determined.
    </para>

    <example id="zend.validate.writing_validators.example.simple">
        <title>Creating a Simple Validation Class</title>

        <para>
            The following example demonstrates how a very simple custom validator might be written.
            In this case the validation rules are simply that the input value must be a floating
            point value.
        </para>

        <programlisting language="php"><![CDATA[
class MyValid_Float extends Zend_Validate_Abstract
{
    const FLOAT = 'float';

    protected $_messageTemplates = array(
        self::FLOAT => "'%value%' is not a floating point value"
    );

    public function isValid($value)
    {
        $this->_setValue($value);

        if (!is_float($value)) {
            $this->_error(self::FLOAT);
            return false;
        }

        return true;
    }
}
]]></programlisting>

        <para>
            The class defines a template for its single validation failure message, which includes
            the built-in magic parameter, <emphasis>%value%</emphasis>. The call to
            <methodname>_setValue()</methodname> prepares the object to insert the tested value into
            the failure message automatically, should the value fail validation. The call to
            <methodname>_error()</methodname> tracks a reason for validation failure.
        </para>
    </example>

    <example id="zend.validate.writing_validators.example.conditions.dependent">
        <title>Writing a Validation Class having Dependent Conditions</title>

        <para>
            The following example demonstrates a more complex set of validation rules, where it is
            required that the input value be numeric and within the range of minimum and maximum
            boundary values. An input value would fail validation for exactly one of the following
            reasons:
        </para>

        <itemizedlist>
            <listitem>
                <para>The input value is not numeric.</para>
            </listitem>

            <listitem>
                <para>The input value is less than the minimum allowed value.</para>
            </listitem>

            <listitem>
                <para>The input value is more than the maximum allowed value.</para>
            </listitem>
        </itemizedlist>

        <para>
            These validation failure reasons are then translated to definitions in the class:
        </para>

        <programlisting language="php"><![CDATA[
class MyValid_NumericBetween extends Zend_Validate_Abstract
{
    const MSG_NUMERIC = 'msgNumeric';
    const MSG_MINIMUM = 'msgMinimum';
    const MSG_MAXIMUM = 'msgMaximum';

    public $minimum = 0;
    public $maximum = 100;

    protected $_messageVariables = array(
        'min' => 'minimum',
        'max' => 'maximum'
    );

    protected $_messageTemplates = array(
        self::MSG_NUMERIC => "'%value%' is not numeric",
        self::MSG_MINIMUM => "'%value%' must be at least '%min%'",
        self::MSG_MAXIMUM => "'%value%' must be no more than '%max%'"
    );

    public function isValid($value)
    {
        $this->_setValue($value);

        if (!is_numeric($value)) {
            $this->_error(self::MSG_NUMERIC);
            return false;
        }

        if ($value < $this->minimum) {
            $this->_error(self::MSG_MINIMUM);
            return false;
        }

        if ($value > $this->maximum) {
            $this->_error(self::MSG_MAXIMUM);
            return false;
        }

        return true;
    }
}
]]></programlisting>

        <para>
            The public properties <varname>$minimum</varname> and <varname>$maximum</varname> have
            been established to provide the minimum and maximum boundaries, respectively, for a
            value to successfully validate. The class also defines two message variables that
            correspond to the public properties and allow <property>min</property> and
            <property>max</property> to be used in message templates as magic parameters, just as
            with <property>value</property>.
        </para>

        <para>
            Note that if any one of the validation checks in <methodname>isValid()</methodname>
            fails, an appropriate failure message is prepared, and the method immediately returns
            <constant>FALSE</constant>. These validation rules are therefore sequentially dependent.
            That is, if one test should fail, there is no need to test any subsequent validation
            rules. This need not be the case, however. The following example illustrates how to
            write a class having independent validation rules, where the validation object may
            return multiple reasons why a particular validation attempt failed.
        </para>
    </example>

    <example id="zend.validate.writing_validators.example.conditions.independent">
        <title>Validation with Independent Conditions, Multiple Reasons for Failure</title>

        <para>
            Consider writing a validation class for password strength enforcement - when a user is
            required to choose a password that meets certain criteria for helping secure user
            accounts. Let us assume that the password security criteria enforce that the password:
        </para>

        <itemizedlist>
            <listitem><para>is at least 8 characters in length,</para></listitem>
            <listitem><para>contains at least one uppercase letter,</para></listitem>
            <listitem><para>contains at least one lowercase letter,</para></listitem>
            <listitem><para>and contains at least one digit character.</para></listitem>
        </itemizedlist>

        <para>
            The following class implements these validation criteria:
        </para>

        <programlisting language="php"><![CDATA[
class MyValid_PasswordStrength extends Zend_Validate_Abstract
{
    const LENGTH = 'length';
    const UPPER  = 'upper';
    const LOWER  = 'lower';
    const DIGIT  = 'digit';

    protected $_messageTemplates = array(
        self::LENGTH => "'%value%' must be at least 8 characters in length",
        self::UPPER  => "'%value%' must contain at least one uppercase letter",
        self::LOWER  => "'%value%' must contain at least one lowercase letter",
        self::DIGIT  => "'%value%' must contain at least one digit character"
    );

    public function isValid($value)
    {
        $this->_setValue($value);

        $isValid = true;

        if (strlen($value) < 8) {
            $this->_error(self::LENGTH);
            $isValid = false;
        }

        if (!preg_match('/[A-Z]/', $value)) {
            $this->_error(self::UPPER);
            $isValid = false;
        }

        if (!preg_match('/[a-z]/', $value)) {
            $this->_error(self::LOWER);
            $isValid = false;
        }

        if (!preg_match('/\d/', $value)) {
            $this->_error(self::DIGIT);
            $isValid = false;
        }

        return $isValid;
    }
}
]]></programlisting>

        <para>
            Note that the four criteria tests in <methodname>isValid()</methodname> do not
            immediately return <constant>FALSE</constant>. This allows the validation class to
            provide <emphasis>all</emphasis> of the reasons that the input password failed to meet
            the validation requirements. if, for example, a user were to input the string "#$%" as a
            password, <methodname>isValid()</methodname> would cause all four validation failure
            messages to be returned by a subsequent call to <methodname>getMessages()</methodname>.
        </para>
    </example>
</sect1>
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