Take care that this function will generate invalid xmlrpc content when invoked with certain parameters (said content will be happily parsed by the lib itself, but not by other implementations).
xmlrpc_encode_request(null, null)
will generate a response without a value
xmlrpc_encode_request('myfunc', array('faultCode' => 666, 'faultString' => 'hello world')
will generated a request containing a <fault> member instead of <params>