recursive function use two foreach

function add(array $arr, $n)
{
    $sum = 0;
    foreach ($arr as $value) {
        if(is_array($value)) {
            $sum += $this->add($value, $n);
        } else {
            $sum += $value;
        }
    }
    
    return $sum;
}

$arr = [1, 2, [3, 4, [5]], 6, [7,8,9]];
$n = 1;
$obj = new add($arr, $n);
$result = $obj->add($arr, $n);
echo $result;