public function host(bool $with_port = true): string
    {
        if (($host = &$this->cacheKey(__FUNCTION__, $with_port)) !== null) {
            return $host; // Cached this already.
        }
        $host = mb_strtolower($_SERVER['HTTP_HOST']);

        if (!$with_port) { // Strip port number?
            $host = preg_replace('/\:[0-9]+$/u', '', $host);
        }
        return $host; // With or without port number.
    }