On the Ibuildings techPortal today there's a great overview of web services from Lorna Mitchell covering everything from the HTTP protocol to data formats to REST and SOAP protocols.

In this world of sharing data, increasing numbers of sites and applications are making information available over web services. Whether we are building a service as a feature of our own development, or pulling in the information published by others, we will need to understand the different service types and how to work with them in PHP. This article aims to give you the tools to do just that.

She starts with the basics every web service developer should know - the HTTP spec - and moves from there into things like RPC services, data formats, SOAP messaging, a few debugging tips and finally one of the more talked about technologies these days - REST.