Description:
------------
Using array_walk_recursive leaves references on source array.

Reproduce code:
---------------
$data = array ('key1' => 'val1', array('key2' => 'val2'), 'key3' =>'val3');

function foo($item, $key){}

var_dump($data);
array_walk_recursive($data,'foo');
var_dump($data);


Expected result:
----------------
array(3) {
  ["key1"]=>
  string(4) "val1"
  [0]=>
  array(1) {
    ["key2"]=>
    string(4) "val2"
  }
  ["key3"]=>
  string(4) "val3"
}
array(3) {
  ["key1"]=>
  string(4) "val1"
  [0]=>
  array(1) {
    ["key2"]=>
    string(4) "val2"
  }
  ["key3"]=>
  string(4) "val3"
}

Actual result:
--------------
array(3) {
  ["key1"]=>
  string(4) "val1"
  [0]=>
  array(1) {
    ["key2"]=>
    string(4) "val2"
  }
  ["key3"]=>
  string(4) "val3"
}
array(3) {
  ["key1"]=>
  string(4) "val1"
  [0]=>
  &array(1) {
    ["key2"]=>
    string(4) "val2"
  }
  ["key3"]=>
  string(4) "val3"
}