Validation function for LUHNMod10 and variant

ISN (International Social Networking Services - Interactive Apps and Websites)

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);

};