public static function unenrol_user($username, $id)
 {
     global $CFG, $DB;
     $params = self::validate_parameters(self::unenrol_user_parameters(), array('username' => $username, 'id' => $id));
     $auth = new auth_plugin_joomdle();
     $id = $auth->unenrol_user($username, $id);
     return $id;
 }