I am currently working on a rather large project which is based on CakePHP framework. In order to simplify the task, I've broken down the whole application into a number of CakePHP plugins. Now, however, I want to enable/disable plugins on the fly. After a brief search around I couldn't find how to do that. Asking a question at #cakephp IRC channel did it. RabidFire instantly replied with the link that gave me an idea. 30 seconds later I had a working solution.

Since the plugins extend the main AppController, all it took was a beforeFilter() method call that looks at a request parameter for the plugin name, checks against an "allowed" array and throws an exception of it's not found or can't load it.

I often use this when implementing a simple login screen for a password protected section of my application. In a Zend Framework application you can implement a preDispatch() function in a Zend_Controller_Action which will run before an action is dispatched. This lets you setup your filter to check to see if the visitor is logged in or not. If the visitor is not logged in, you can redirect them to the login screen of your application.

He includes example code to show its usefulness - checking to see two things: is a user is logged in and to see if they're allowed to use a certain resource.