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

If you'd like to view the request without actually making a connection, you can override SoapClient's __doRequest method to return the XML:

class DummySoapClient extends SoapClient {
    function __construct($wsdl, $options) {
        parent::__construct($wsdl, $options);
    }
    function __doRequest($request, $location, $action, $version, $one_way = 0) {
        return $request;
    }
}
$SOAP = new DummySoapClient('http://example.com/?wsdl', array('trace' => true));
echo $SOAP->GetRequestDetail($params);

$SOAP = new SoapClient($WSDL, array('trace' => true));

$Response = $SOAP->DoRemoteFunction($Data);

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

This will not only print the last request but also make the XML tags visible in the browser.

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));

Extending Quinn's answer, you can also just log the request before you perform the request.

class SoapClientDebug extends SoapClient
{

public function __doRequest($request, $location, $action, $version, $one_way = 0)
{
    error_log("REQUEST:\n" .$request . "\n");
    error_log("LOCATION:\n" .$location . "\n");
    error_log("ACTION:\n" .$action . "\n");
    error_log("VERSION:\n" .$version . "\n");
    error_log("ONE WAY:\n" .$one_way . "\n");

    return parent::__doRequest($request, $location, $action, $version, $one_way);
}
}