max solution

<?php

function my_max_internal_compare($x, $max)
{
    return ($x > $max ? $x : $max);
}

function my_max_internal($x0, $max)
{
    if (is_null($x0)) return $max;

    if (!is_array($x0)) return my_max_internal_compare($x0, $max);

    if (count($x0) < 1) return $max;

    foreach ($x0 as $x)
    {
        $max = my_max_internal($x, $max);
    }

    return $max;
}

function my_max($xs)
{
    if (is_null($xs) || count($xs) < 1) return 0;

    $max = -2147483648; //PHP_INT_MIN

    foreach ($xs as $x0)
    {
        $max = my_max_internal($x0, $max);
    }

    return $max;
}

echo my_max([1, 2, [3, [1000, 4, 5], 6, 7], 8, 9]) . "\n"; // 1000