[RESOLVED] What's wrong here?

[Zach_VB6]

The assignment:

The prime factors of 13195 are 5, 7, 13 and 29.

What is the largest prime factor of the number 600851475143 ?

My code:

<center>

<?PHP

    function isPrime($num) {
        for ($i = $num-1; $i > 1; $i--)
            if ($num % $i == 0) return false;
        return true;
    }

    $num = 600851475143;
    
    for ($x = $num-1; $x > $num; $x--) 
        if (($num % $x == 0) && isPrime($x))
            die(strval($x));
?>

</center>

It just shows a white screen...

Edit: the &# 37 is a % in my code... not sure why it's doing that