In some of his development work, Chris Hartjes has come across two tools that have made his life simpler, one that's specifically related to PHP and building APIs.
Well folks, I've had another one of these ["why didn't I see this earlier"] moments. What is it? Repeat after me: THOU SHALL BUILD YOUR APPLICATION AS AN API FIRST, AND FRONT END SECOND. [...] So, here are two tools that I think my readers should know about. For those working with PHP, I cannot recommend enough the use of FRAPI if you have a legacy application that you need to provide an API for.
He talks a bit about what FRAPI is and how it can help lay right on top of your current application to create a full-featured API without much of the hand-coding hassle. It does a lot of the work for you, generating the files needed and handling REST requests right out of the box.
The documentation via the wiki isn't that great, but it gives you everything you really need to get started. Kudos to the boys at echolibre for creating a super-useful tool.