Switchtrack

Switchtrack provides a framework for developing object oriented, RESTful APIs in PHP. It leverages a router that provides a clean interface between your client-side Javascript framework and database resources in your server environment.

Put another way, Switchtrack lets you express an API endpoint like http://www.yoursite.com/api/user/20/getdisplayname as a PHP class, where the service name ('user') is the class name, and the endpoint ('getdisplayname') is a method of the class. Switchtrack automatically turns the response data into JSON, which your client side Javascript can then consume.