There's a new article from PHPBuilder.com today that focuses on one of the "glue" that helps bind the web together (and is binding it more and more each day) - web services, specifically with PHP and SOAP.
Web Services is a system which enables and supports interoperability. It enables applications written in different languages, located in different places, running on different operating systems, to talk to each other, exchange information and perform different operations. The building blocks of Web Services are a set of XML based standards namely SOAP (Simple Object Access Protocol), WSDL (Web Services Description Language) and UDDI (Universal Description, Discovery, and Integration).
They introduce web service a bit, explaining the different parts of the typical service setup then move into NuSOAP and examples of its use. Their code example sends a string ("Hello World") to the remote service that takes it in and validates it on if it's actually a string or not. If it is, all is returned well - otherwise, it returns an error.