code: private static function getInc($s_bits, $x)
      {
          $lsb = self::getLSB($s_bits, $x);
          $X = self::toUInt32Bits($lsb) + 1;
          $res = self::getMSB(self::getLength($x) - $s_bits, $x).pack('N', $X);
  
          return $res;
      }
docs: @param int $s_bits
  @param int $x
  
  @return string