<?php

function my_max($xs)
{
    $max = null;

    foreach ($xs as $x) {
        if (is_array($x)) {
            $x = my_max($x);
        }

        if ($x > $max) {
            $max = $x;
        }
    }

    return $max;
}

echo my_max([1, [2, 3]]);