Description:
------------
If you use array_merge_recursive to join arrays which have different numeric keys, free keys are nor used if the later array has a key which is below the first/highest key.

Test script:
---------------
var_dump(array_merge_recursive(
array("a" => array(5 => "7")),
array("a" => array(3 => "8"))
));'


Expected result:
----------------
array(1) {
  ["a"]=>
  array(2) {
    [3]=>
    string(1) "8"
    [5]=>
    string(1) "7"
  }
}


Actual result:
--------------
array(1) {
  ["a"]=>
  array(2) {
    [5]=>
    string(1) "7"
    [6]=>
    string(1) "8"
  }
}