Suggestions of GPL plugin-system?

Hi guys, I've been developing a GPL PHP/MySQL app for some time now and I would like to extend it with a module/plugin system. The idea would be that people could add a directory in a plugin path that would contain a bunch of PHP files extending the functionality of my application. This directory would then be read out, some config file parsed and whatnot, after which the module can be turned on by my application. Now, I could try and figure this out by myself, but that would be reinventing the wheel since I'm betting there is some good GPL modular software around (such as Joomla, PHP-Nuke, PHPbb, etc, etc.) that you have been working with as a coder. Could any of you suggest a certain GPL application that has a great module setup that I could take a look at? Thanks a lot for your time! Ivo