Php > Authentication

Validation function for LUHNMod10 and variant. Can discriminate credit card

Validation function for LUHNMod10 and variant. Can discriminate credit card
numbers of varying lengths.

<? 
function prestoLUHN($ccn="",$alt="0") 
{     
// pre-validations 
$ccn = ereg_replace("[^[:digit:]]+", "", $ccn); 
if ((strlen($ccn) < 1)) return FALSE; 

// Mapping: Double-then-SumOfDigits  (0123456789) => (0246813579) 
for($i=0;$i<=9;$i++) $v[$i] = (($i*2)<9) ? ($i*2):($i*2)-9; 

// Initial value of $alt (Alternation Pattern) determines which digits are transformed by $v[] during checksum calculation 
// 0 = apply transform to even-numbered digits from LSD-to-MSD (Conventional LUHNMod10) 
// 1 = apply transform to  odd-numbered digits from LSD-to-MSD (alternative) 

$checksum = 0;      
// Calculate checksum from the end of the string (the low-order digit) to beginning of string 
for($i = strlen($ccn)-1; $i >= 0; $i--) 
{ 
   $digit = substr($ccn, $i, 1); 
   $checksum += $alt ? $v[$digit] : $digit; 
   $alt ^= 1; 
} 

// return the checksum descrimination result 
return !($checksum % 10);       
};          
?>