Symfony.
After having tried at least 10 others over a course of 5 years or so developing in PHP, none of the old traditional frameworks like CakePHP, CodeIgniter, etc. are doing nearly as much innovation as Symfony, and do not step up to the plate when it comes to enterprise-level performance demands. I'm totally foregoing discussions about Symfony's advantages in scalability, modularity, ORMs and command-line tools. I'll let you dive into those on your own.
Having said that, since you're just now getting started with this type of thing I'd suggest trying them all just as a learning experience.
Another thing you should probably look into ASAP if you're doing web development is Ruby and RoR. Ruby is already becoming a stronger alternative to PHP in a number of situations. If it weren't for Symfony, I'd probably be using Ruby exclusively, out of necessity.
Sidenote : Alot of people will recommend Zend. Ignore them, this will save you lots of time and frustration.