How do I see the actual XML generated by PHP SOAP Client Class?

Use getLastRequest. It returns the XML sent in the last SOAP request.

echo "REQUEST:\n" . $SOAP->__getLastRequest() . "\n";

And remember, this method works only if the SoapClient object was created with the trace option set to TRUE. Therefore, when creating the object, use this code:

$SOAP = new SoapClient($WDSL, array('trace' => 1));