Why does array_map() with null as callback create an "array of arrays"?

php arrays specifications array-map

PHP Snippet 1:

$d = array_map('array', $a, $b, $c);

PHP Snippet 2:

$array = [
    [1 => 'one', 2 => 'two', 3 => 'three']
];
var_export(array_map(fn() => func_get_args(), ...$array));
echo "\n---\n";
var_export(array_map(null, ...$array));

PHP Snippet 3:

array (
  1 => 
  array (
    0 => 'one',
  ),
  2 => 
  array (
    0 => 'two',
  ),
  3 => 
  array (
    0 => 'three',
  ),
)
---
array (
  1 => 'one',
  2 => 'two',
  3 => 'three',
)