Andrew Podner has an introductory post to one of the more popular PHP frameworks (and one that's been around for a long time) - CodeIgniter.

I decided to pick CodeIgniter back up and start really trying to learn the ins and outs of it. I was pleasantly surprised that overall, it is pretty simple to use and whenever you want to do something, the first question to ask yourself should be "Let me check the user guide and see if there is a library or helper for this before I write it". Chances are, the answer has already been thought of and exists in the framework.

The post is pretty high level and describes the parts of the MVC structure (Model/View/Controller) and how they relate to the features that CodeIgniter has to offer. For more information about this framework (and downloads) check out codeigniter.com.