The devReview.com website has put together The Big List of as many of the PHP frameworks out there they could find:

For the start of a new year, it was time to clean out the list of PHP frameworks. A few have fallen by the wayside, but many new ones have been added. It seems that patterns and MVC is still all the rage, but less are professing to being Rails clones, though the inspiration of convention is still strong. There are a wide variety of framework types. From full stack (cakePHP, Symfony) to components (eZ), a bit of both (Zend) and minimalist (LightVC, TinyMVC, Pluf).

Requirements to be on the list include that the framework must be PHP-based, have a recent code release in the last 12 months, needs to be available in English and not including frameworks that can be considered content systems themselves.

As of the time of this post, there are around 50 active frameworks listed - everything from CakePHP and the Zend Framework out to eZ Components and Prado. Links for each include their project homepage and some reviews the site has done.