function is_valid_credit_card($s) {
$s = strrev(preg_replace('/[^\d]/','',$s));
$sum = 0;
for ($i = 0, $j = strlen($s); $i < $j; $i++) {
if (($i % 2) == 0) {
$val = $s[$i];
} else {
$val = $s[$i] * 2;
if ($val > 9) { $val -= 9; }
}
$sum += $val;
}
return (($sum % 10) == 0);
}
if (! is_valid_credit_card($_POST['credit_card'])) {
print 'Sorry, that card number is invalid.';
}
?>