(Yaf >=1.0.0)
Plugins allow for easy extensibility and customization of the framework.
Plugins are classes. The actual class definition will vary based on the component -- you may need to implement this interface, but the fact remains that the plugin is itself a class.
A plugin could be loaded into Yaf by using Yaf_Dispatcher::registerPlugin(), after registerd, All the methods which the plugin implemented according to this interface, will be called at the proper time.
Example #1 Plugin example
<?php
   /* bootstrap class should be defined under ./application/Bootstrap.php */
   class Bootstrap extends Yaf_Bootstrap_Abstract {
        public function _initPlugin(Yaf_Dispatcher $dispatcher) {
            /* register a plugin */
            $dispatcher->registerPlugin(new TestPlugin());
        }
   }
   /* plugin class should be placed under ./application/plugins/ */
   class TestPlugin extends Yaf_Plugin_Abstract {
        public function routerStartup(Yaf_Request_Abstract $request, Yaf_Response_Abstract $response) {
            /* before router 
               in this hook,  user can do some url rewrite */
            var_dump("routerStartup");
        }
        public function routerShutdown(Yaf_Request_Abstract $request, Yaf_Response_Abstract $response) {
            /* router complete 
               in this hook, user can do login check */
            var_dump("routerShutdown");
        }
        public function dispatchLoopStartup(Yaf_Request_Abstract $request, Yaf_Response_Abstract $response) {
            var_dump("dispatchLoopStartup");
        }
        public function preDispatch(Yaf_Request_Abstract $request, Yaf_Response_Abstract $response) {
            var_dump("preDispatch");
        }
        public function postDispatch(Yaf_Request_Abstract $request, Yaf_Response_Abstract $response) {
            var_dump("postDispatch");
        }
        public function dispatchLoopShutdown(Yaf_Request_Abstract $request, Yaf_Response_Abstract $response) {
            /* final hoook
               in this hook user can do loging or implement layout */
            var_dump("dispatchLoopShutdown");
        }
   }
   Class IndexController extends Yaf_Controller_Abstract {
        public function indexAction() {
            return FALSE; //prevent rendering
        }
   }
   $config = array(
       "application" => array(
           "directory" => dirname(__FILE__) . "/application/",
       ),
   );
 
   $app = new Yaf_Application($config);
   $app->bootstrap()->run();
?>
The above example will output something similar to:
string(13) "routerStartup" string(14) "routerShutdown" string(19) "dispatchLoopStartup" string(11) "preDispatch" string(12) "postDispatch" string(20) "dispatchLoopShutdown"
$request
   , Yaf_Response_Abstract $response
   )