In this recent post to his blog Ian Selby looks at how you can create a REST API that can be used as an interface layer on top of any PHP application.

One of the latest (sort of) crazes sweeping the net is APIs, more specifically those that leverage REST. It's really no surprise either, as consuming REST APIs is so incredibly easy… in any language. It's also incredibly easy to create them as you essentially use nothing more than an HTTP spec that has existed for ages.

He looks at what REST is - both the request and response sides - and some of the PHP code you can use to create your own service. His example defines all of the HTTP codes and uses a processRequest function to handle the incoming request. The response is pushed back out from the processResponse method and even includes an extension of the class to handle XML requests if they're POSTed.