php crc16

function crc16($string, $crc = 0)
{
    for ($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;
}