3
there are a couple ready to use packages for consuming / serving SOAP services in PHP
PEAR::SOAPNuSOAPI personally have used the PEAR::SOAP implementation and found its OO design very nice.
In related news, I recently purchased the PHP Anthology Books from Sitepoint.com and I found them to be a wonderful set of training reference books for someone learning the language. If you already know PHP, book 2 still has several good chapters, covering topics like XML, XML-RPC, SOAP, Caching, Access Control mechanism, etc.
http://www.sitepoint.com/books/phpant1/