array keys as values with multidimensional array

Question

I currently have an array looking like this

Array
(
    [7] => Array
        (
            [49] => 
            [41] => 
        )

    [8] => Array
        (
            [70] => 
            [69] => 
        )

    [105] => 
    [9] => 
    [10] => 
)

Now, I need each to work with each key, but I struggle getting those through a foreach() loop, because there is no values. I have been trying to use array_keys, but that disregards multidimensional keys. Is there a way I can assign the keys as the values as well, to have a structure like this?

Array
(
    [7] => Array
        (
            [49] => 49
            [41] => 41
        )

    [8] => Array
        (
            [70] => 70
            [69] => 69
        )

    [105] => 105
    [9] => 9
    [10] => 10
)

This way I could use a foreach() to get the values of each key. I don't know if this is the easiest way, but I would love to get some hints into the right direction. Thanks!

Answer 1

Use array_walk_recursive() to assign the key as value to each entry whose value is not an array.

Answer 2

From PHP7.4, you can enjoy the brevity of "arrow function" syntax. You still need to make the leafnode's value modifiable by reference and manually assign $k's value to $v. The arrow function syntax attempts (by default) to return the value in the expression, but the return value is ignored.

Code: (Demo)

$array = [
    7 => [49 => null, 41 => null],
    8 => [70 => null, 69 => null],
    105 => null,
    9 => null,
    10 => null
];

array_walk_recursive($array, fn(&$v, $k) => $v = $k);
var_export($array);

Output:

array (
  7 => 
  array (
    49 => 49,
    41 => 41,
  ),
  8 => 
  array (
    70 => 70,
    69 => 69,
  ),
  105 => 105,
  9 => 9,
  10 => 10,
)