<?php

function my_max($array)
{
    foreach($array as $key => $value) {
        if (is_array($value)) {
            $array[$key] = my_max($value);
        }
    }

    $maxNumber = $array[0];

    for($j = 0; $j < count($array); $j++){
        if($maxNumber < $array[$j]){
            $maxNumber = $array[$j];
        }
    }

    return $maxNumber;
}

echo my_max([1, 2, [3, [1000, 4, 5], 6, 7], 8, 9])

?>