The "cool kids" in PHP are using Laravel 4. It's a well made framework. I'm doing a REST API in it at work and the lack of mind share and docs is probably the worst part about it (it's still in beta to be fair). Most of the time I ended up reading the framework code.
PHP leaves a lot to be desired though. It boggles my mind how developers who love what they do have the patience to stick with it. Example: Feature request for the unless control structure: https://bugs.php.net/bug.php?id=40296
I've seen a few people doing it. The API is quite stable since the end of last year or so I've heard. I didn't start using it until recently and I haven't encountered any bugs and whenever I gear up to do a pull request someone else has already done it.
We used Symfony 2 on projects previously. I find that it makes simple things hard to do and it is too verbose.
Laravel 4 uses many of the Symfony 2 components but makes nice facades that you can call as static methods so you never have to worry about namespace importing and it makes things a lot less verbose and easy to use.
PHP leaves a lot to be desired though. It boggles my mind how developers who love what they do have the patience to stick with it. Example: Feature request for the unless control structure: https://bugs.php.net/bug.php?id=40296