Here is a tested and working CRC16-Algorithm:

<?php
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;
}
?>

Regards,
Mario