DevShed.com has published the next (and last) installment in their Building a Site Engine with PHP. Part 5 takes all of the structure that you've built up so far (including the plugin system) and shows you how to put them into practice.

They work primarily with the plugin system in this article, wrapping up the most flexible part of the content management system. With a combination of PHP, MySQL, and a little bit of patience, adding plugins, modules, and/or blocks can be a snap.

Here are the links to the entire series: Part 1, Part 2, Part 3, Part 4, and, of course, Part 5.