Как преобразовать многомерный php массив в одномерный

<?php

$collection = [['hexlet', ['is', 'awesome']], ['1', 2], [3, [4, 5, 6]]];

$result = [];
array_walk_recursive($collection, function($item) use (&$result) {
    $result[] = $item
});
print_r($result);
// => Array
// => (
// =>     [0] => hexlet
// =>     [1] => is
// =>     [2] => awesome
// =>     [3] => 1
// =>     [4] => 2
// =>     [5] => 3
// =>     [6] => 4
// =>     [7] => 5
// =>     [8] => 6
// => )