function my_max(array $values): int {
    $max = $values[0];
    foreach ($values as $currentVal) {
        if (is_array($currentVal)) {
            $currentVal = my_max($currentVal);
        }
        if ($max < $currentVal) {
            $max = $currentVal;
        }
    }
    return $max;
}

echo my_max([1, 2, 3, [5, 7]]);
exit;