According to Akash Mehta in this new post on the SitePoint PHP blog, all of the frameworks out there aren't really needed because "last we checked, PHP IS a framework".
Now, consider the "average" PHP frameworks. They help you handle request data, manage your output, control app flow - essentially, extending PHP's inbuilt functionality. They are, therefore, PHP frameworks on the PHP framework. PHP provides a vast array (pardon the pun) of functionality out of the box. But when you want to do things your way, it gets out of the way, and this is really important when building anything beyond a simple database frontend.
He suggests that PHP is a "half framework" on its own, providing quick an easy methods and functionality to build up applications. He gives an example of Twitter (and really, who isn't picking on them these days) and how it probably could have avoided a lot of the issues it is seeing if it had gone with something a bit more efficient and flexible.
Be sure to check out the comments for some other great opinions from the community.