private static function getProxyString(): ?string
    {
        $proxyString = null;
        if (\getenv('HTTP_CLIENT_IP')) {
            $proxyString = \getenv('HTTP_CLIENT_IP');
        } elseif (\getenv('HTTP_X_FORWARDED_FOR')) {
            $proxyString = \getenv('HTTP_X_FORWARDED_FOR');
        } elseif (\getenv('HTTP_X_FORWARDED')) {
            $proxyString = \getenv('HTTP_X_FORWARDED');
        } elseif (\getenv('HTTP_X_COMING_FROM')) {
            $proxyString = \getenv('HTTP_X_COMING_FROM');
        } elseif (\getenv('HTTP_FORWARDED_FOR')) {
            $proxyString = \getenv('HTTP_FORWARDED_FOR');
        } elseif (\getenv('HTTP_FORWARDED')) {
            $proxyString = \getenv('HTTP_FORWARDED');
        } elseif (\getenv('HTTP_COMING_FROM')) {
            $proxyString = \getenv('HTTP_COMING_FROM');
        }
        return $proxyString;
    }