public function isEngine(): bool
    {
        if (($is = &$this->cacheKey(__FUNCTION__)) !== null) {
            return $is; // Cached this already.
        }
        $user_agent = $_SERVER['HTTP_USER_AGENT'] ?? '';
        return $is  = $user_agent && preg_match('/(?:blink|gecko|konqueror|msie|opera|playstation|presto|trident|webkit)/ui', $user_agent);
    }