public function isSsl(): bool
    {
        if (($is = &$this->cacheKey(__FUNCTION__)) !== null) {
            return $is; // Cached this already.
        }
        if (!empty($_SERVER['SERVER_PORT'])) {
            if ((int) $_SERVER['SERVER_PORT'] === 443) {
                return $is = true;
            }
        }
        if (!empty($_SERVER['HTTPS'])) {
            if (filter_var($_SERVER['HTTPS'], FILTER_VALIDATE_BOOLEAN)) {
                return $is = true;
            }
        }
        if (!empty($_SERVER['HTTP_X_FORWARDED_PROTO'])) {
            if ($this->c::mbStrCaseCmp($_SERVER['HTTP_X_FORWARDED_PROTO'], 'https') === 0) {
                return $is = true;
            }
        }
        return $is = false;
    }