public static function resolve_url($url)
    {
        // prepend protocol://hostname:port
        if (!preg_match('|^https?://|', $url)) {
            $schema       = 'http';
            $default_port = 80;

            if (self::https_check()) {
                $schema       = 'https';
                $default_port = 443;
            }

            $prefix = $schema . '://' . preg_replace('/:\d+$/', '', $_SERVER['HTTP_HOST']);
            if ($_SERVER['SERVER_PORT'] != $default_port) {
                $prefix .= ':' . $_SERVER['SERVER_PORT'];
            }

            $url = $prefix . ($url[0] == '/' ? '' : '/') . $url;
        }

        return $url;
    }