PHP SOAP: error: looks like we got no XML document

This error has driven me crazy for hours.

The solution is much easier than it sounds:

The SOAP Client in PHP expects plain XML without any control characters.
In my case I had 2 trailing newline character in one of the PHP files which were included during the initialization of the soap server.

I just removed all trailing characters after the '?>' in all PHP files and everything worked perfectly.

More information about SOAP Webservices: 
PHP : SOAP: Creating and Consuming a WSDL Web-Service