SOAP is a lightweight protocol for exchange of information in a decentralized, distributed environment. To get some ideas of SOAP using with PHP have a look at [1][2]. Bruce Schneier talking about security issues on SOAP [3].
[1] http://www.phpbuilder.com/columns/luis20010329.php3?page=1
[2] http://dietrich.ganx4.com/nusoap/index.php
[3] http://www.counterpane.com/crypto-gram-0006.html
