<?php
$num = "49927398716";

$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) {
        echo "Invalid\n";
} else {
        echo "Valid\n";
}