CakePHP? CodeIgniter? Symfony? Other?
eisokant: I have only tried CodeIgniter so I can only speak for that but it is by far the best decision I have made to move to this MVC framework. It's holds a lot of libraries and helpers which will simplify most tasks and they are incredibly easy to extend. It's downside though is that you're not able to call functions internally from other controllers but once you get used to that it doesn't pose many problems. I would recommend you having a look at their documentation (which is very well done) and a demo video.