public static function create_username($consumerkey, $ltiuserid) {
        if (!empty($ltiuserid) && !empty($consumerkey)) {
            $userkey = $consumerkey . ':' . $ltiuserid;
        } else {
            $userkey = false;
        }

        return 'enrol_lti' . sha1($consumerkey . '::' . $userkey);
    }