public function calcMod10V1(string $number, string $separator = "", int $minimumLength = 6): string
    {
        $revstr = strrev((string) intval($number));
        $revstrLen = strlen($revstr);
        $total = 0;
        for ($i = 0; $i < $revstrLen; $i++) {
            if ($i % 2 == 0) {
                $multiplier = 2;
            } else {
                $multiplier = 1;
            }

            $subtotal = intval($revstr[$i]) * $multiplier;
            if ($subtotal >= 10) {
                $temp = (string) $subtotal;
                $subtotal = intval($temp[0]) + intval($temp[1]);
            }
            $total += $subtotal;
        }

        $checkDigit = (10 - ($total % 10)) % 10;
        $crn = str_pad(ltrim($number, "0"), $minimumLength - 1, "0", STR_PAD_LEFT) . $separator . $checkDigit;

        return $crn;
    }