The controller architecture includes a plugin system that allows user code to be called when certain events occur in the controller process lifetime. The front controller uses a plugin broker as a registry for user plugins, and the plugin broker ensures that event methods are called on each plugin registered with the front controller.

The event methods are defined in the abstract class Zend_Controller_Plugin_Abstract, from which user plugin classes inherit:

In order to write a plugin class, simply include and extend the abstract class Zend_Controller_Plugin_Abstract:

<?php
require_once 'Zend/Controller/Plugin/Abstract.php';

class MyPlugin extends Zend_Controller_Plugin_Abstract
{
    // ...
}
?>

None of the methods of Zend_Controller_Plugin_Abstract are abstract, and this means that plugin classes are not forced to implement any of the available event methods listed above. Plugin writers may implement only those methods required by their particular needs.

Zend_Controller_Plugin_Abstract also makes the request and response objects available to controller plugins via the getRequest() and getResponse() methods, respectively.

Plugin classes are registered with Zend_Controller_Front::registerPlugin(), and may be registered at any time. The following snippet illustrates how a plugin may be used in the controller chain:

<?php
require_once 'Zend/Controller/Front.php';
require_once 'Zend/Controller/Router.php';
require_once 'Zend/Controller/Plugin/Abstract.php';

class MyPlugin extends Zend_Controller_Plugin_Abstract
{
    public function routeStartup()
    {
        $this->getResponse()->appendBody("<p>routeStartup() called</p>\n");
    }

    public function routeShutdown($request)
    {
        $this->getResponse()->appendBody("<p>routeShutdown() called</p>\n");
    }

    public function dispatchLoopStartup($request)
    {
        $this->getResponse()->appendBody("<p>dispatchLoopStartup() called</p>\n");
    }

    public function preDispatch($request)
    {
        $this->getResponse()->appendBody("<p>preDispatch() called</p>\n");
    }

    public function postDispatch($request)
    {
        $this->getResponse()->appendBody("<p>postDispatch() called</p>\n");
    }

    public function dispatchLoopShutdown()
    {
        $this->getResponse()->appendBody("<p>dispatchLoopShutdown() called</p>\n");
    }
}

$front = Zend_Controller_Front::getInstance();
$front->setControllerDirectory('/path/to/controllers')
      ->setRouter(new Zend_Controller_Router_Rewrite())
      ->registerPlugin(new MyPlugin());
$front->dispatch();

Assuming that no actions called emit any output, and only one action is called, the functionality of the above plugin would still create the following output:

<p>routeStartup() called</p>
<p>routeShutdown() called</p>
<p>dispatchLoopStartup() called</p>
<p>preDispatch() called</p>
<p>postDispatch() called</p>
<p>dispatchLoopShutdown() called</p>

	Note
	Plugins may be registered at any time during the front controller execution. However, if an event has passed for which the plugin has a registered event method, that method will not be triggered.

On occasion, you may need to unregister or retrieve a plugin. The following methods of the front controller allow you to do so:

Zend Framework includes a plugin for error handling in its standard distribution.

<?php
class ErrorController extends Zend_Controller_Action
{
    public function errorAction()
    {
        $errors = $this->_getRequest('error_handler');
    }
}
?>

<?php
class ErrorController extends Zend_Controller_Action
{
    public function errorAction()
    {
        $errors = $this->_getRequest('error_handler');

        switch ($errors->type) {
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_CONTROLLER:
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_ACTION:
                // 404 error -- controller or action not found
                $this->getResponse()->setRawHeader('HTTP/1.1 404 Not Found');

                // ... get some output to display...
                break;
            default:
                // application error; display error page, but don't change
                // status code
                break;
        }
    }
}
?>

<?php
require_once 'Zend/Controller/Front.php';
require_once 'Zend/Controller/Plugin/ErrorHandler.php';

$front = Zend_Controller_Front::getInstance();
$front->registerPlugin(new Zend_Controller_Plugin_ErrorHandler());
?>

<?php
require_once 'Zend/Controller/Front.php';
require_once 'Zend/Controller/Plugin/ErrorHandler.php';

$front = Zend_Controller_Front::getInstance();
$front->registerPlugin(new Zend_Controller_Plugin_ErrorHandler(array
    'module'     => 'mystuff',
    'controller' => 'static',
    'action'     => 'error'
)));
?>

<?php
require_once 'Zend/Controller/Front.php';
require_once 'Zend/Controller/Plugin/ErrorHandler.php';

$plugin = new Zend_Controller_Plugin_ErrorHandler();
$plugin->setErrorHandlerModule('mystuff')
       ->setErrorHandlerController('static')
       ->setErrorHandlerAction('error');

$front = Zend_Controller_Front::getInstance();
$front->registerPlugin($plugin);
?>

<?php
class ErrorController extends Zend_Controller_Action
{
    public function errorAction()
    {
        $errors = $this->_getRequest('error_handler');

        switch ($errors->type) {
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_CONTROLLER:
            case Zend_Controller_Plugin_ErrorHandler::EXCEPTION_NO_ACTION:
                // 404 error -- controller or action not found
                $this->getResponse()->setRawHeader('HTTP/1.1 404 Not Found');

                $content =<<<EOH
<h1>Error!</h1>
<p>The page you requested was not found.</p>
EOH;
                break;
            default:
                // application error
                $content =<<<EOH
<h1>Error!</h1>
<p>An unexpected error occurred with your request. Please try again later.</p>
EOH;
                break;
        }
        $this->initView();
        $this->view->content = $content;
        $this->render();
    }
}
?>