First, let's assume the following directory structure and class files, and that the toplevel directory and library directory are on the include_path:

application/
    modules/
        foo/
            views/
                helpers/
                    FormLabel.php
                    FormSubmit.php
        bar/
            views/
                helpers/
                    FormSubmit.php
library/
    Zend/
        View/
            Helper/
                FormLabel.php
                FormSubmit.php
                FormText.php

Now, let's create a plugin loader to address the various view helper repositories available:

<?php
$loader = new Zend_Loader_PluginLoader();
$loader->addPrefixPath('Zend_View_Helper', 'Zend/View/Helper/')
       ->addPrefixPath('Foo_View_Helper', 'application/modules/foo/views/helpers')
       ->addPrefixPath('Bar_View_Helper', 'application/modules/bar/views/helpers');
?>

We can then load a given view helper using just the portion of the class name following the prefixes as defined when adding the paths:

<?php
// load 'FormText' helper:
$formTextClass = $loader->load('FormText'); // 'Zend_View_Helper_FormText';

// load 'FormLabel' helper:
$formLabelClass = $loader->load('FormLabel'); // 'Foo_View_Helper_FormLabel' 

// load 'FormSubmit' helper:
$formSubmitClass = $loader->load('FormSubmit'); // 'Bar_View_Helper_FormSubmit' 
?>

Once the class is loaded, we can now instantiate it.

Note: Multiple paths may be registered for a given prefix
In some cases, you may use the same prefix for multiple paths. Zend_Loader_PluginLoader actually registers an array of paths for each given prefix; the last one registered will be the first one checked. This is particularly useful if you are utilizing incubator components.

Note: Paths may be defined at instantiation
You may optionally provide an array of prefix / path pairs (or prefix / paths -- plural paths are allowed) as a parameter to the constructor:

<?php
$loader = new Zend_Loader_PluginLoader(array(
    'Zend_View_Helper' => 'Zend/View/Helper/',
    'Foo_View_Helper'  => 'application/modules/foo/views/helpers',
    'Bar_View_Helper'  => 'application/modules/bar/views/helpers'
));
?>

Zend_Loader_PluginLoader also optionally allows you to share plugins across plugin-aware objects, without needing to utilize a singleton instance. It does so via a static registry. Indicate the registry name at instantiation as the second parameter to the constructor:

<?php
// Store plugins in static registry 'foobar':
$loader = new Zend_Loader_PluginLoader(array(), 'foobar');
?>

Other components that instantiate the PluginLoader using the same registry name will then have access to already loaded paths and plugins.

The example in the previous section shows how to add paths to a plugin loader. What if you want to determine the paths already loaded, or remove one or more?

• getPaths($prefix = null) returns all paths as prefix / path pairs if no $prefix is provided, or just the paths registered for a given prefix if a $prefix is present.

• clearPaths($prefix = null) will clear all registered paths by default, or only those associated with a given prefix, if the $prefix is provided and present in the stack.

• removePrefixPath($prefix, $path = null) allows you to selectively remove a specific path associated with a given prefix. If no $path is provided, all paths for that prefix are removed. If a $path is provided and exists for that prefix, only that path will be removed.

Sometimes you simply want to determine if a plugin class has been loaded before you perform an action. isLoaded() takes a plugin name, and returns the status.

Another common use case for the PluginLoader is to determine fully qualified plugin class names of loaded classes; getClassName() provides this functionality. Typically, this would be used in conjunction with isLoaded():

<?php
if ($loader->isLoaded('Adapter')) {
    $class   = $loader->getClassName('Adapter');
    $adapter = call_user_func(array($class, 'getInstance'));
}
?>