Question

I am using the following PHP code to calculate a CRN for BPay:

<?php
function LuhnCalc($number) {
  $chars = array_reverse(str_split($number, 1));
  $odd = array_intersect_key($chars, array_fill_keys(range(1, count($chars), 2), null));
  $even = array_intersect_key($chars, array_fill_keys(range(0, count($chars), 2), null));
  $even = array_map(function($n) { return ($n >= 5)?2 * $n - 9:2 * $n; }, $even);
  $total = array_sum($odd) + array_sum($even);
  return ((floor($total / 10) + 1) * 10 - $total) % 10;
}
print LuhnCalc($_GET['num']);
?>

However it seems that BPAY is version 5 of MOD 10, for which I can’t find any documentation. It seems to not be the same as MOD10.

The following numbers where tested:

2005,1597,3651,0584,9675

bPAY
2005 = 20052
1597 = 15976
3651 = 36514
0584 = 05840
9675 = 96752

MY CODE 
2005 = 20057 
1597 = 15974 
3651 = 36517 
0584 = 05843 
9675 = 96752

As you can see, none of them match the BPAY numbers.

Answer 1

This PHP function will generate BPay reference numbers based on the mod10 version 5 algorithm.

Who knows why BPay can’t add this to their website. I only found an explanation by googling finding the algorithm being called “MOD10V05” instead of “Mod 10 version 5”.

function generateBpayRef($number) {

    $number = preg_replace("/\D/", "", $number);

    // The seed number needs to be numeric
    if(!is_numeric($number)) return false;

    // Must be a positive number
    if($number <= 0) return false;

    // Get the length of the seed number
    $length = strlen($number);

    $total = 0;

    // For each character in seed number, sum the character multiplied by its one based array position (instead of normal PHP zero based numbering)
    for($i = 0; $i < $length; $i++) $total += $number{$i} * ($i + 1);

    // The check digit is the result of the sum total from above mod 10
    $checkdigit = fmod($total, 10);

    // Return the original seed plus the check digit
    return $number . $checkdigit;

}