On this site Matt Sparks has posted the first few parts of a series covering the creation of a custom framework. Why? Well, as he explains in part one of the series:

So with all of that being said, it begs the question: why on Earth would you want to do this?

The extremely short answer: I want to. The less short answer: A PHP framework encompasses many of the areas I want to learn more about.

The first four posts of the series are already on his site (with more to come):

• Part 1 – Why? Seriously, Why?
• Part 2 – What is a Web Framework?
• Part 3 – Time For Action
• Part 4 – The Foundation

Matt does a great job of laying out some of the fundamentals behind frameworks including structure, design patterns, and commonalities between frameworks. You can follow along with his progress on the project on the AnalyzePHP GitHub repositories.