Luhn algorithm for validating credit cards

The Luhn algorithm also known as the “modulus 10″ or “mod 10″ algorithm, is a checksum formula which can be used to validate credit card numbers. Developed in the 1950’s by IBM scientist Hans Peter Luhn and described in U.S. Patent 2,950,048. A PHP implementation is shown below.


function LuhnCheck($strDigits)
{
$sum = 0;
$alt = false;
for($i = strlen($strDigits) - 1; $i >= 0; $i--)
{
if($alt)
{
$temp = $strDigits[$i];
$temp *= 2;
$strDigits[$i] = ($temp > 9) ? $temp = $temp - 9 : $temp;
}
$sum += $strDigits[$i];
$alt = !$alt;
}
return $sum % 10 == 0;
}

?>