Recently I had to put up a SOAP web service with PHP. This was my first contact with Soap and Wsdl.

Some links that got me started:
http://www.php.net/soap
http://www.devshed.com/c/a/Zend/PHP-SOAP-Extension/
http://www.w3.org/TR/wsdl
http://www.w3schools.com/wsdl/wsdl_intro.asp
http://xml.coverpages.org/wsdl.html

To test the service i used SoapUI.