prudp.php

<?php
function prudp_calculate_checksum($hex) {
    $hex_dump = str_split($hex, 2); // split hex string into an array every 2 parts
    var_dump($hex_dump);
    $checksum_result = 1; // 1 because the final output of the checksum is a bit off
    foreach ($hex_dump as &$value) { // foreach hex stuff
        $checksum_result += hexdec($value); // sum up of all the hex in decimal
    }
    $checksum_result = $checksum_result % 256; // mod by 256, this is where the magic begins
    return dechex($checksum_result); // this should probably be hex?
}

$packet_uncalculated = 'afa140' . str_repeat('00', 12);
$packet_checksum = prudp_calculate_checksum($packet_uncalculated);
$packet_calculated = $packet_uncalculated . $packet_checksum;
echo($packet_calculated);