This function works exactly the same as xmlrpc_decode() except it gives you access to the method being called as well (xmlrpc_decode() only outputs the params from the XML-RPC call).
To use this function, simply create an empty variable to store the method string in, and then pass by reference like so:
$method = null;
$params = xmlrpc_decode_request('<xml rpc request>', &$method);
What you will get back are the params stored in $params, and $method will have the method being requested.