ParagonIE_Sodium_Core_Util::hex2bin

    public static function hex2bin($hexString, $strictPadding = false)
    {
        /* Type checks: */
        if (!is_string($hexString)) {
            throw new TypeError('Argument 1 must be a string, ' . gettype($hexString) . ' given.');
        }

        /** @var int $hex_pos */
        $hex_pos = 0;
        /** @var string $bin */
        $bin = '';
        /** @var int $c_acc */
        $c_acc = 0;
        /** @var int $hex_len */
        $hex_len = self::strlen($hexString);
        /** @var int $state */
        $state = 0;
        if (($hex_len & 1) !== 0) {
            if ($strictPadding) {
                throw new RangeException(
                    'Expected an even number of hexadecimal characters'
                );
            } else {
                $hexString = '0' . $hexString;
                ++$hex_len;
            }
        }

        $chunk = unpack('C*', $hexString);
        while ($hex_pos < $hex_len) {
            ++$hex_pos;
            /** @var int $c */
            $c = $chunk[$hex_pos];
            /** @var int $c_num */
            $c_num = $c ^ 48;
            /** @var int $c_num0 */
            $c_num0 = ($c_num - 10) >> 8;
            /** @var int $c_alpha */
            $c_alpha = ($c & ~32) - 55;
            /** @var int $c_alpha0 */
            $c_alpha0 = (($c_alpha - 10) ^ ($c_alpha - 16)) >> 8;
            if (($c_num0 | $c_alpha0) === 0) {
                throw new RangeException(
                    'hex2bin() only expects hexadecimal characters'
                );
            }
            /** @var int $c_val */
            $c_val = ($c_num0 & $c_num) | ($c_alpha & $c_alpha0);
            if ($state === 0) {
                $c_acc = $c_val * 16;
            } else {
                $bin .= pack('C', $c_acc | $c_val);
            }
            $state ^= 1;
        }
        return $bin;
    }