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

Today I learned about a special case of array_map() in PHP, which is mentioned as a side note in the documentation:

Example #4 Creating an array of arrays

<?php
$a = array(1, 2, 3, 4, 5);
$b = array("one", "two", "three", "four", "five");
$c = array("uno", "dos", "tres", "cuatro", "cinco");

$d = array_map(null, $a, $b, $c);
print_r($d);
?>

The above example will output:

Array
(
    [0] => Array
        (
            [0] => 1
            [1] => one
            [2] => uno
        )

    [1] => Array
        (
            [0] => 2
            [1] => two
            [2] => dos
        )

    [2] => Array
        (
            [0] => 3
            [1] => three
            [2] => tres
        )

    [3] => Array
        (
            [0] => 4
            [1] => four
            [2] => cuatro
        )

    [4] => Array
        (
            [0] => 5
            [1] => five
            [2] => cinco
        )

)

If the array argument contains string keys then the returned array will contain string keys if and only if exactly one array is passed. If more than one argument is passed then the returned array always has integer keys.

(try it out)

But that’s it. No more explanation. I understand, that this does the same as

$d = array_map(function() { return func_get_args(); }, $a, $b, $c);

But why would anybody want or expect this as default behavior? Is there a technical reason why it works like that, like a side effect from the implemtation? Or was this just a random “let’s make this function do one more thing” decision (looking at you, array_multisort())?

Answer

This appears to be a special case in _array_map_, but it’s only documented in that example. NULL is not normally allowed as a callback (if you try to use it with call_user_func() it reports an error), but it’s allowed in _array_map()_. It treats NULL as meaning that it should simply create an array of the arguments.

This is useful because array is also not valid as a callback, because it’s language construct, not a function. So you can’t write:

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