Some of the best documentation on using SOAP with PHP is quite well hidden.  If you like the convenience of using PEAR packages you should check out these docs (download zip).

Also, a good article out a few days ago on using Services_Google, the PEAR package for using the Google API.

Anyone using Overture’s DTC Advertiser Web Service?  They have made the unusual choice of offering a web service with no SOAP API.