private function validateTemporaryTaxpayer(string $vatNumber): bool
    {
        if ($vatNumber[10] != 1) {
            return false;
        }

        $weights = [1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 2];
        $checksum = (int)$vatNumber[11];
        $checkval = $this->sumWeights($weights, $vatNumber);

        if (($checkval % 11) == 10) {
            $weights = [3, 4, 5, 6, 7, 8, 9, 1, 2, 3, 4];
            $checkval = $this->sumWeights($weights, $vatNumber);
            $checkval = ($checkval % 11 == 10) ? 0 : $checkval % 11;

            return $checkval == $checksum;
        }

        return $checkval % 11 == $checksum;
    }