I've been using the ICMP Checksum calculation function written by Khaless [at] bigpond [dot] com. But when having an odd length of data, it failed, so I made my own instead, which adds a 0 if the data length is odd:
<?php
function icmpChecksum($data)
{
// Add a 0 to the end of the data, if it's an "odd length"
if (strlen($data)%2)
$data .= "\x00";
// Let PHP do all the dirty work
$bit = unpack('n*', $data);
$sum = array_sum($bit);
// Stolen from: Khaless [at] bigpond [dot] com
// The code from the original ping program:
// sum = (sum >> 16) + (sum & 0xffff); /* add hi 16 to low 16 */
// sum += (sum >> 16); /* add carry */
// which also works fine, but it seems to me that
// Khaless will work on large data.
while ($sum>>16)
$sum = ($sum >> 16) + ($sum & 0xffff);
return pack('n*', ~$sum);
}
?>