Frameworks are made for easing the application development and deployment. What you need to focus on when working with a framework is your logic, instead of the "behind code" programming. Plus, you don't have to repair the core code, you only need to repair YOUR code if something goes wrong (perhaps ZF has some inner bugs, but they can be easily fixed if you know how it works). Of course, learning a framework can be a little tricky, but one you get the basics, it all becomes very easy. However, I would only recommend Zend Framework for developing large scale applications. It works extremely well and is fully compatible with PHP5. Besides that, it has a large amount of already made classes and a built-in helpers system. I don't see CodeIgniter (not even CakePHP) as a true framework, because the pattern they follow involve too much learning and sometimes it can be really hard to accomplish what you want to achieve. Even ZF has its flaws and sometimes needs some hacking, but atm it's the best PHP framework to use.

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