Generate 16 digit number that passes Luhn algorithm

Question

I'm trying to generate 16 digit number that passes the Luhn algorithm verification. For example, If I'm generating 16 digit number for an American Express card which would begin with 37

Something like this

3789 1477 0763 171 (this would pass the Luhn algorithm)

I able to generate the 16 digit number but it wouldn't pass the Luhn algorithm.

<?php 

function generatenumber($limit, $prefix){
   $code = '';
   for($i = 0; $i < $limit; $i++) { 
       $code .= mt_rand(0, 9); 
   }
   return $prefix.$code;
}

generatenumber(14,37);

3711414458103430  // This wouldn't pass the Luhn algorithm verification)

?>

Answer 1

Here is an attempt you can try:

<?php

  function generate_numbers($limit, $prefix) {
    $digits = substr(str_shuffle(str_repeat('0123456789', $limit)), 0, $limit - 1);

    $sum = 0;
    foreach (str_split(strrev($digits)) as $i => $digit) {
      $sum += ($i % 2 == 0) ? array_sum(str_split($digit * 2)) : $digit;
    }

    return $prefix . $digits . (10 - ($sum % 10)) % 10;
  }