How to add a new array to the very end of a multidimensional array of arbitrary depth in php?

Question

PHP

There is an array of arbitrary depth (up to several hundred nestings), where each element of the array is either another filled array or an empty array. But always an array!
There is a function that bypasses this array, and:
If it finds an empty array "at the edge" (at the bottom) of the depth of a multidimensional array, then it puts another array there.
If it has skidded, then it scrolls further, if it has not skidded and the depth no longer stops.
Array example:

$test_mass['tk98op'] = array('p5ld2se'=>array(),
     '38hjp9'=>array('2we8t2'=>array(), '1142ln'=>array()),
     '669yo36'=>array('6318u7'=>array('22rv86'=>array(),
     '13we42'=>array()), 'jk66d3'=>array()));

Here is my code:

function deep($mass, $b = 0)
    {
    foreach ($mass as $key=>$val)
        {
        if(is_array($val))
            {
            if(count($val) == 0)
                {
                /* не один из этих трех вариантов ничего не дописывает */
                //$val = array(1,4,217,432);
                //$mass = array_merge($val, array(1,4,217,432));
                //$mass[$key] = array(1,4,217,432);  
                }     
            deep($val, $b);
            }
        }
    return $mass;
    }

As I suppose, the problem is in full addressing to the depth, but I still don’t understand how to solve it universally. Please help.
Here is what we should have as an output:

Array
(
    [tk98op] => Array
        (
            [p5ld2se] => Array
                (
                    [0] => 1
                    [1] => 4
                    [2] => 217
                    [3] => 432
                )
            [38hjp9] => Array
                (
                    [2we8t2] => Array
                        (
                            [0] => 1
                            [1] => 4
                            [2] => 217
                            [3] => 432
                        )
                    [1142ln] => Array
                        (
                            [0] => 1
                            [1] => 4
                            [2] => 217
                            [3] => 432
                        )
                )
            [669yo36] => Array
                (
                    [6318u7] => Array
                        (
                            [22rv86] => Array
                                (
                                    [0] => 1
                                    [1] => 4
                                    [2] => 217
                                    [3] => 432
                                )
                            [13we42] => Array
                                (
                                    [0] => 1
                                    [1] => 4
                                    [2] => 217
                                    [3] => 432
                                )
                        )
                    [jk66d3] => Array
                        (
                            [0] => 1
                            [1] => 4
                            [2] => 217
                            [3] => 432
                        )
                )
        )
)

PS I also do not really understand if my function will bypass a multidimensional array, for example, with two (three, four, etc.) deep branches?

Answer 1

1. If you want to change an element in the array, then you need to specify a reference to it &$val
2. When you run recursion and you need the data that it returns, then you need to write it down $val = deep($val);
3. You can’t foreachuse it array_merge
Here’s what happened:

$test_mass = [
    'tk98op' => [
        'p5ld2se' => [],
        '38hjp9'  => [
            '2we8t2' => [],
            '1142ln' => [],
        ],
        '669yo36' => [
            '6318u7' => [
                '22rv86' => [],
                '13we42' => [],
            ],
            'jk66d3' => [],
        ],
    ],
];

function deep($mass)
{
    foreach ($mass as $key => &$val) {
        $val = (\is_array($val) && \count($val) === 0)
            ?
            [1, 4, 217, 432]
            : deep($val);
    }
    unset($val);

    return $mass;
}