This section of code validates a credit card or any other luhn number - useful as an initial card number check for merchant interfaces etc

the function returns either 0 or 1 depending on whether the luhn check was successful. The luhn check does a load of recursive calculation and then divides by 10 and checks if there is a remainder

input length 16

1. function luhnCheck($number){
3. $sum = 0;
4. $alt = false;
6. for($i = strlen($number) - 1; $i >= 0; $i--){
7. $n = substr($number, $i, 1);
8. if($alt){
9. //square n
10. $n *= 2;
11. if($n > 9) {
12. //calculate remainder
13. $n = ($n % 10) +1;
14. }
15. }
16. $sum += $n;
17. $alt = !$alt;
18. }
19. //echo $sum;
20. //if $sum divides by 10 with no remainder then it's valid
21. return ($sum % 10 == 0);
22. }