SOAP (Simple Object Access Protocol) is a lightweight XML-based protocol for exchanging structured information between distributed applications over native web protocols, such as HTTP. SOAP specifies the formats that XML messages should use, the way in which they should be processed, a set of encoding rules for standard and application-defined data types, and a convention for representing remote procedure calls and responses.

Web Services is a modern and very popular technology. The list of protocols and technologies related to Web Services grows every day, but SOAP is probably the most important. It is rapidly becoming the standard protocol for accessing Web Services. It uses XML messages to exchange information across endpoints, and provides several advantages over other binary protocols.  RPC (Remote Procedure Calls) support was originally a minor element in the design of SOAP, but this feature is one of the most useful it has today.

PHP 5’s SOAP extension is the first attempt to implement the SOAP protocol for PHP in C. It has some advantages over the existing implementations written in PHP itself, the main one being speed. The extension is currently marked as experimental, but should gradually become more stable and reliable as time progresses.

The SOAP extension implements a large subset of SOAP 1.1, SOAP 1.2 and WSDL 1.1 specifications. The key goal is to use the RPC feature of the SOAP protocol. WSDL is used where possible in order to make the implementation of Web Services more straightforward.