On O'Reilly's ONLamp.com website, there's a new tutorial by Deepak Vohra about the creation of web services with PHP, two different kinds - a SOAP service (and client) and an XML-RPC web service.
A web service consists of a server to serve requests to the web service and a client to invoke methods on the web service. The PHP class library provides the SOAP extension to develop SOAP servers and clients and the XML-RPC extension to create XML-RPC servers and clients. Before I delve further into developing web services with PHP, I shall briefly discuss web services.
HE starts with the SOAP service, using the functionality from the php_soap module to create a simple SOAP server (including the creation of a WSDL file, example included). The other side of things is included as well - a SOAP client that makes a request for items from a certain catalog and outputs to a page.
He uses the XML-RPC functionality to make the same type of service (with the same data) and includes the request and response XML for their request for the "hello" message the server responds with.