We use CakePHP. Our MVP was hacked from Drupal, but we trashed that quickly due to performance issues with our project management system.
I think there are plenty of startups using PHP. It just isn't quite as trendy, so you don't hear about it as much. There are many reasons to pick PHP over Ruby/Python. PHP and its frameworks tend to be quicker to start working with, and PHP often executes faster (sometimes much faster) than Ruby or Python ever will. Apps that need to scale would probably do better to use PHP/HipHop than Django or Rails. Plus, there are definitely more PHP programmers out there than Python/Ruby, so there is the added benefit of having a larger pool of talent to hire from.