On the PHPMaster.com site there's a recent tutorial posted, the first part in a series of posts from David Shirey about building REST APIs in PHP. This first part of the series stays pretty high-level and really just introduces some common REST concepts.
Thousands of years ago when we first started building web pages, things were very simple. You’d put some text on the page, maybe even an image, and that was pretty much it. But today it’s a whole different ball game. Instead of static pages there’s the dynamic applications we’ve come to depend on. And so, how these applications are designed to communicate becomes very important. In this series I’ll introduce you to the REST architecture style. In this article I’ll help you to understand exactly what it is, and later I’ll show you how it can be implemented in a PHP environment.
He defines the term "REST" for those not familiar and how a typical RESTful API allows other end users/software to interact directly with its data. He outlines some of the common principles of REST and finishes the post with a comparison of two HTTP verbs - PUT and POST.