Like everything else, there's no best. It's down to personal preferences. Yii looks new and fresh. Zend looks like a leviathan. Mind you I got a nosebleed from Zend a few years ago. These aren't the only frameworks.
yes, there are a lot of framework, its also truth that we can't learn all the framework, so which one we should learn, zend, cakephp, symphony or yii which is the latest and review show its the most advance.
Go for Yii if you're just starting up. It has a very active community and comes bundled with very in-depth guide, blog tutorial and API reference. CodeIgniter is also one of my favorites, although I personally prefer symfony for big projects.
You should go with a reputable one like Zend. Something that is going to continue to update years from now and stay with that one. You don't want to end up in a situation where you are trying to update php or apache and have it break, just to find out that if you want to update you have to move your entire business to a new framework. Either that or build your own so that you will know what parts of the code need to be modified in order to work with the latest versions of php or apache.