private static function calculateChecksumMod10($content) {
        // Calculating Checksum
        // Consider the right-most digit of the message to be in an "odd" position,
        // and assign odd/even to each character moving from right to left
        // Odd Position = 3, Even Position = 1
        // Multiply it by the number
        // Add all of that and do 10-(?mod10)
        $odd = true;
        $checksumValue = 0;
        $c = strlen($content);

        for ($i = $c; $i > 0; $i--) {
            if ($odd === true) {
                $multiplier = 3;
                $odd = false;
            } else {
                $multiplier = 1;
                $odd = true;
            }

            $checksumValue += ($content[$i - 1] * $multiplier);
        }

        return (10 - $checksumValue % 10) % 10;
    }