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

today learned special case of array_map() in php, mentioned side note in documentation:

example #4 creating 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 output:

array (     [0] => array         (             [0] => 1             [1] => 1             [2] => uno         )      [1] => array         (             [0] => 2             [1] => 2             [2] => dos         )      [2] => array         (             [0] => 3             [1] => 3             [2] => tres         )      [3] => array         (             [0] => 4             [1] => 4             [2] => cuatro         )      [4] => array         (             [0] => 5             [1] => 5             [2] => cinco         )  ) 

if array argument contains string keys returned array contain string keys if , if 1 array passed. if more 1 argument passed returned array has integer keys.

(try out)

but that's it. no more explanation. understand, same as

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

but why want or expect default behavior? there technical reason why works that, side effect implemtation? or random "let's make function 1 more thing" decision (looking @ you, array_multisort())?

this appears special case in _array_map_, it's documented in example. null not allowed callback (if try use call_user_func() reports error), it's allowed in _array_map()_. treats null meaning should create array of arguments.

this useful because array not valid callback, because it's language construct, not function. can't write:

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