I have attached an example of how to accomplish the SOAP communication via PHP using CURL. The example is from a real implementation of how to simulate a WebLink login.
Regarding books / training / technology:
While the attached example uses CURL, I have personally used NuSoap/PHP to accomplish the same thing - NuSoap. As jhigginbotham stated this is XML over HTTP so you can use many programming flavors - C#, VB.NET, ASP script, JavaScript, Perl, Python, Java, and PHP.
The following is a great article that explains in detail how the two technologies work together - XML.com.
I am currently working on some client libraries to ease DataExchange implementation for .NET, Perl, PHP and JavaScript users.
--Nick Floyd