Description:
------------
While array_change_key_case allows for changing array keys in regards to capitalization, it is very limited in its function.
Therefore I request the introduction of a new function (for the sake of the discussion, I'll call it "array_change_keys") that takes an array and a callback and then calls the callback for each key, passing the key as a parameter (possibly also passing the value as another parameter) and replaces the old key with the return value of the function call - under the same constraints as mentioned in http://php.net/array_change_key_case in terms of replacing elements with colliding keys.

Signature:
array array_change_keys  ( array $input, callback $callback )

Reproduce code:
---------------
<?php

$a = array(
  'peter' => 'm, 71',
  'paul'  => 'm, 72',
  'mary'  => 'f, 73'
);

$b = array_change_keys($a, 'strlen');


Expected result:
----------------
$b === array(
  5 => 'm, 71',
  4 => 'f, 73'
)

('paul' was overridden by the element 'mary' which resulted in the same key name)