Description:
------------
The functions array_shift and array_pop should get an optional second parameter, taking a reference to a variable in which the key of the shifted/popped element will be stored.

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

$a = array(
  'foo' => 'bar',
  'qux' => 'quux'
);

$value = array_shift($a, $key);
echo "$key: $value";


Expected result:
----------------
foo: bar

Actual result:
--------------
array_shift does not take a second parameter at the moment.