<?php
function roundup ($number, $increments) {
        $increments = 1 / $increments;
        return (ceil($number * $increments) / $increments);
}

@@$num = "49927398717";

$double = false;
$ret = "";

for ($i = strlen($num) - 1; $i >= 0; $i--) {
        $ret .= ($double ? $num[$i] * 2 : $num[$i]);
        $double = !$double;
}
$ret = strrev($ret);
$sum = 0;

for ($i = 0; $i < strlen($ret); $i++) {
        $sum += $ret[$i];
}
echo "\$ret = $ret\n";
echo "\$sum = $sum\n";

if ($sum % 10) {
        $check_digit = roundup($sum, 10) - $sum;
        echo "Invalid - (Valid with check digit $check_digit)\n";
} else {
        echo "Valid\n";
}
?>