The best PHP framework?

Hi, all:

I'd like to start developing my applications with PHP. I think a good 
framework would save costs, but I wonder which is the best one.

I've read Zend Framework is probably the most complete in terms of 
functionality, but I've read as well it is quite slow in performance. On the 
other hand, I've found a framework called Yii, which is presented as an 
"extremely fast PHP framework"; but I'm not sure it is as complete as I need 
(with MVC, office and .pdf documents generation, with an easy integration 
with mobile devices or SMS platforms, data encryption, etc...).

I'd really appreciate any insight from all of you with experience in these 
frameworks (Yii and Zend).

Thanks a lot in advance.