rest9 is a PHP RESTful API framework that allows you to quickly set up an API.
All you need to do is to create classes with all the custom logic code needed.
Please note that all the public methods in your classes will be callable from the API, so be sure that the only public methods are those that have "harmless" code.
rest9 is still in early alpha. It has allot of optimization problems and I'm currently rewriting some horrendous pieces of code.
To install rest9 you need to place the code in a LAMP server. There's a .htaccess included that needs to be in the rest9 default folder for it to work.