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);
};
?>
