Lee Blue has posted something looking to bridge yet another gap between Ruby on Rails and PHP - a method for adding before_filter functionality to PHP.
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.