Zend is the one I use most, but isn't to easy to learn. Also CakePHP is a nice one. But will can choose the right framework after you work with all or some of them.
I have tried code igniter and cakephp. Much prefered code igniter. No experience really in PHP but using the documentation and forum managed to create everything I wanted so far.
There are alot of good php frameworks. CakePHP and codeigniter are well documented. I would suggest to choose one of these mature frameworks. They are easy to set up and do some quick testing and see for yourself which one suits you best.