I'd also recommend Kohana. It was originally a fork of CodeIgniter, but version 3.0 was a complete rewrite for PHP5. Compared to Symfony, it's more of a barebones framework.
I wouldn't. Kohana's track record of cavalier response to security concerns is off-putting and their code is not particularly impressive.
There's a reason people who use PHP are getting on @fabpot's train and not Kohana's, and that's because Symfony2 does much, much more right than anyone else in the conversation.
