SOAP is an XML-based protocol for messaging and RPC-style communication between two processes. The World Wide Web Consortium's XML Protocol (XP) Working Group is working on a SOAP standard, which will be called XP.
The SOAPX4 (Simple Object Access Protocol) implementation for PHP is at http://dietrich.ganx4.com/soapx4.
Alternate SOAP implementation is at http://sourceforge.net/projects/phpxmlp.