Description:
------------
array_walk_recursive() modifies element values deeper than the first level on vars outside the scope of the function call.



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

$ar = array(
    'element 1',
    array('element2 => subelement1')
    );

func($ar);
var_dump($ar);

function func($a) {
  array_walk_recursive($a, 'apply');
}

function apply(&$input, $key) {
  $input = 'changed';
}


Expected result:
----------------
array(2) {
  [0]=>
  string(9) "element 1"
  [1]=>
  array(1) {
    [0]=>
    string(23) "element2 => subelement1"
  }
}


Actual result:
--------------
array(2) {
  [0]=>
  string(9) "element 1"
  [1]=>
  array(1) {
    [0]=>
    string(7) "changed"
  }
}