On the PHP in Action blog this new post talks about something that's at the core of the front controller for most frameworks - a call to a user function based on the passed in action.

The core of your average web framework is a Front Controller. Front Controllers are commonly considered complex and esoteric. That's a myth. I sometimes brag that I can construct a Front Controller in 15 minutes. Actually, it's doesn't take quite that long. In PHP, a Front Controller can be simplified to just one line of code.

This one line of code, while a very dangerous thing to actually use in an application, illustrates what a front controller does to forward out the request to the rest of the framework. He revises it with a Zend Framework-ish example that splits the request out into a controller/action method.