$callback	$arr	array_keys_map($callback, $arr)
'strtoupper'	Array ( [d] => ddd [e] => eee [f] => fff [0] => g )	Array ( [D] => ddd [E] => eee [F] => fff [0] => g )
function ($key) { return $key * $key; }	Array ( [1] => hhh [2] => iii [3] => jjj [4] => g )	Array ( [1] => hhh [4] => iii [9] => jjj [16] => g )

Array
(
    [0] => a
    [1] => b
    [8] => c
    [27] => d
)
Array
(
    [A] => 1
    [B] => 2
    [C] => 3
    [D] => 4
)