Why those nearly the same codes get a different result in php's array_walk + ksort?

Question

I want to sort a multi-array by key, those data come form front end somehow will be mess up in an random sort.

So I must make the data sorted, at first I natural use the combine 'array_walk($array, "ksort")'. Work fine at simple array data, while dealing a multi-array, it work wrong.

Finally, I solve it by loop ksort.

But I have no clue why the nearly same codes get a different result.

Someone help? Thx

    $arr = [
        ['identifier_num' => 1, 'form_id' => 3, 'is_view' => 1, 'in_time' => '3', 'model_num' => 44],
        ['form_id' => 1, 'identifier_num' => 3, 'in_time' => 1, 'is_view' => '3', 'model_num' => 44],
        ['b' => 1, 'a' => '2'],
    ];
    $a2 = $arr;

    array_walk($arr, 'diydiy');
    var_dump($arr);

    array_walk($a2, 'ksort');
    var_dump($a2);

    function diydiy(&$a)
    {
        ksort($a);
    }

The result:

array (size=3)
  0 => 
    array (size=5)
      'form_id' => int 3
      'identifier_num' => int 1
      'in_time' => string '3' (length=1)
      'is_view' => int 1
      'model_num' => int 44
  1 => 
    array (size=5)
      'form_id' => int 1
      'identifier_num' => int 3
      'in_time' => int 1
      'is_view' => string '3' (length=1)
      'model_num' => int 44
  2 => 
    array (size=2)
      'a' => string '2' (length=1)
      'b' => int 1
D:\WWW\t.php:13:
array (size=3)
  0 => 
    array (size=5)
      'form_id' => int 3
      'identifier_num' => int 1
      'in_time' => string '3' (length=1)
      'is_view' => int 1
      'model_num' => int 44
  1 => 
    array (size=5)
      'model_num' => int 44
      'is_view' => string '3' (length=1)
      'in_time' => int 1
      'identifier_num' => int 3
      'form_id' => int 1
  2 => 
    array (size=2)
      'a' => string '2' (length=1)
      'b' => int 1