function crc16($string) {
  $crc = 0xFFFF;
  for ($x = 0; $x < strlen ($string); $x++) {
   $crc = $crc ^ ord($string[$x]);
   for ($y = 0; $y < 8; $y++) {
     if (($crc & 0x0001) == 0x0001) {
       $crc = (($crc >> 1) ^ 0xA001);
     } else { $crc = $crc >> 1; }
   }
  }
  return $crc;
}