public function getID(): ?string
    {
        $sessionKey = $this->getSessionKey();
        if (strlen($sessionKey) === 0) {
            return null;
        }
        $cacheKey = 'id-' . $sessionKey;
        if (!isset(self::$cache[$cacheKey])) {
            self::$cache[$cacheKey] = null;
            $data = Internal\Data::getValue('.temp/bearcms/userkeys/' . md5($sessionKey));
            if ($data !== null) {
                self::$cache[$cacheKey] = $data;
            }
        }
        return self::$cache[$cacheKey];
    }