array flatten

flatten_example.php

<?php
function flatten($input) {
    $return = new StdClass();
    $return->flat = [];
    array_walk_recursive(
        $input,
        function($item, $key, $userdata) {
            $userdata->flat[] = $item;
        },
        $return
    );
    return $return->flat;
}
print_r(flatten([1,[2,3],4,[[5],6]]));