I'm delighted to announce that my new book "PHP Web Services" is now available as an early release! [...] The book isn't huge (or expensive, hint!), but it aims to give solid theory in a practical and approachable way. There's the topics you'd expect to see, covering HTTP and verbs and headers and status codes, and also around data formats. It also covers RPC services including SOAP, and also has a chapter (predictably the longest one!) about REST. I've tried to go beyond simply the "how to do" and into the "how to do in a kick-ass manner" realm, so there are chapters about how to design your API and choose what kind to build, how to handle errors, how to make your API really robust - and of course how to debug when things go wrong!
The book not only has the summaries and descriptions of some common web service challenges, but also includes code samples you can use in your own projects.