Making a CMS extendable by plugins - examples

I'm working on an open source script. I would like to provide a feature to extend the script by use of plugins/modules.

I need some recommendations on open source cms/scripts to study code from, to gather the best practices on making script extendable by plugins. I've used Wor开发者_开发百科dPress in the past, and was planning to implement a similar system, but I'm looking to dive into alternatives, which might be better.

Language preferred is PHP 5

Well Drupal(http://drupal.org/) is another open source CMS widely used in industry with quite a few plugins for it. Since it's open source you can just go through the source code related to the plugin aspects of it and see how they do it. Another one to look at would be Joomla(http://www.joomla.org/)

You could always check out Drupal.