I\'ve recently begun building version 2 of my year-old custom MVC framework. It\'s worked great for me on a number of projects, but I definitely see room for improvement. It
I have developed my own framework by extending ZF. And I can tell you that it can be easily done. You can use what you want, and replace what you dont want quite easily. Full ZF follows OOP to the core. They use best practices everywhere. I have learned a lot by observing its code.
Personally I think ZF is the best php framework I have ever seen.