SOAP, the Simple Object Access Protocol, is the powerhouse of web services. It's a highly adaptable, object-oriented protocol that exists in over 80 implementations on every popular platform, including .NET, JavaScript, and PHP. It provides a flexible communication layer between applications, regardless of platform and location. As long as they both speak SOAP, a PHP-based web application can ask a C++ database application on another continent to look up the price of a book and have the answer right away.

SOAP was created collaboratively as an open protocol. Early in its development, XML-RPC was spun off, and now enjoys its own popularity as a simpler alternative to SOAP. Both encode messages as XML, and both use HTTP to transport those messages. SOAP, however, can use other transport protocols, offers a number of high-end features, and is developing rapidly.

A SOAP transaction begins with an application making a call to a remote procedure. The SOAP client script then encodes the procedure request as an XML payload and sends it over the transport protocol to a server script. The server parses the request and passes it to a local method, which returns a response. The response is encoded as XML by the server and returned as a response to the client, which parses the response and passes the result to the original function.