# Placeholder Helper

The `Placeholder` view helper is used to persist content between view scripts and view instances. It also offers some useful features such as aggregating content, capturing view script content for later use, and adding pre- and post-text to content (and custom separators for aggregated content).

## Basic Usage of Placeholders

Basic usage of placeholders is to persist view data. Each invocation of the `Placeholder` helper expects a placeholder name; the helper then returns a placeholder container object that you can either manipulate or simply echo out.

    <?php $this->placeholder('foo')->set("Some text for later") ?>

    <?php
        echo $this->placeholder('foo');
        // outputs "Some text for later"
    ?>

## Using Placeholders to Aggregate Content

Aggregating content via placeholders can be useful at times as well. For instance, your view script may have a variable array from which you wish to retrieve messages to display later; a later view script can then determine how those will be rendered.

The `Placeholder` view helper uses containers that extend `ArrayObject`, providing a rich featureset for manipulating arrays. In addition, it offers a variety of methods for formatting the content stored in the container:

- `setPrefix($prefix)` sets text with which to prefix the content. Use `getPrefix()` at any time to determine what the current setting is.

- `setPostfix($prefix)` sets text with which to append the content. Use `getPostfix()` at any time to determine what the current setting is.

- `setSeparator($prefix)` sets text with which to separate aggregated content. Use `getSeparator()` at any time to determine what the current setting is.

- `setIndent($prefix)` can be used to set an indentation value for content. If an integer is passed, that number of spaces will be used; if a string is passed, the string will be used. Use `getIndent()` at any time to determine what the current setting is.

    <!-- first view script -->
    <?php $this->placeholder('foo')->exchangeArray($this->data) ?>

    <!-- later view script -->
    <?php
    $this->placeholder('foo')->setPrefix("<ul>\n    <li>")
                             ->setSeparator("</li><li>\n")
                             ->setIndent(4)
                             ->setPostfix("</li></ul>\n");
    ?>

    <?php
        echo $this->placeholder('foo');
        // outputs as unordered list with pretty indentation
    ?>

Because the `Placeholder` container objects extend `ArrayObject`, you can also assign content to a specific key in the container easily, instead of simply pushing it into the container. Keys may be accessed either as object properties or as array keys.

    <?php $this->placeholder('foo')->bar = $this->data ?>
    <?php echo $this->placeholder('foo')->bar ?>

    <?php
    $foo = $this->placeholder('foo');
    echo $foo['bar'];
    ?>

## Using Placeholders to Capture Content

Occasionally you may have content for a placeholder in a view script that is easiest to template; the `Placeholder` view helper allows you to capture arbitrary content for later rendering using the following API.

- `captureStart($type, $key)` begins capturing content.

  `$type` should be one of the `Placeholder` constants `APPEND` or `SET`. If `APPEND`, captured content is appended to the list of current content in the placeholder; if `SET`, captured content is used as the sole value of the placeholder (potentially replacing any previous content). By default, `$type` is `APPEND`.

  `$key` can be used to specify a specific key in the placeholder container to which you want content captured.

  `captureStart()` locks capturing until `captureEnd()` is called; you cannot nest capturing with the same placeholder container. Doing so will raise an exception.

- `captureEnd()` stops capturing content, and places it in the container object according to how `captureStart()` was called.

    <!-- Default capture: append -->
    <?php $this->placeholder('foo')->captureStart();
    foreach ($this->data as $datum): ?>
    <div class="foo">
        <h2><?php echo $datum->title ?></h2>
        <p><?php echo $datum->content ?></p>
    </div>
    <?php endforeach; ?>
    <?php $this->placeholder('foo')->captureEnd() ?>

    <?php echo $this->placeholder('foo') ?>

    <!-- Capture to key -->
    <?php $this->placeholder('foo')->captureStart('SET', 'data');
    foreach ($this->data as $datum): ?>
    <div class="foo">
        <h2><?php echo $datum->title ?></h2>
        <p><?php echo $datum->content ?></p>
    </div>
     <?php endforeach; ?>
    <?php $this->placeholder('foo')->captureEnd() ?>

    <?php echo $this->placeholder('foo')->data ?>

## Concrete Placeholder Implementations

Zend Framework ships with a number of "concrete" placeholder implementations. These are for commonly used placeholders: doctype, page title, and various <head> elements. In all cases, calling the placeholder with no arguments returns the element itself.

Documentation for each element is covered separately, as linked below:

- Doctype

- HeadLink

- HeadMeta

- HeadScript

- HeadStyle

- HeadTitle

- InlineScript