PHP Luhn Checker - validates credit cards etc

2008-06-09

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

1. function luhnCheck($number){ 
2.     
3.    $sum = 0; 
4.    $alt = false; 
5.     
6.    for($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. }