Some years ago, Rasmus Lerdorf, original creator of PHP, posted on his personal Web site a discussion of what he would look for in a framework for the MVC architecture. It’s an interesting read, from the master’s voice. He specifically talks about how to properly use MVC, how to guarantee performance and security, and how to design with scalability and flexibility in mind. Although the post is a few years old, most of it is still valid, although there are little changes such as the fact that the PECL/Filter extension is now part of PHP proper.
Lerdorf also posted his own thoughts on Facebook’s use of HipHop PHP for faster PHP execution, if you’re looking for something to read.