Description:
------------
When we are using array_fill function with negative $start_index it produces wrong array result.
As I see this is documented behaviour (http://php.net/manual/en/function.array-fill.php#function.array-fill.example.basic ).

Also there is note in documentation: "See also the Arrays section of manual for a detailed explanation of negative keys."
But there is no any information about negative keys in arrays and why it should work like this.

We can support negative indexes in arrays now that's why it's better to make array_fill work as expected.

Test script:
---------------
var_dump(array_fill_keys(range(-2,1), true));
var_dump(array_fill(-2, 4, true))

Expected result:
----------------
array(4) {
  [-2]=>
  bool(true)
  [-1]=>
  bool(true)
  [0]=>
  bool(true)
  [1]=>
  bool(true)
}
array(4) {
  [-2]=>
  bool(true)
  [-1]=>
  bool(true)
  [0]=>
  bool(true)
  [1]=>
  bool(true)
}

Actual result:
--------------
array(4) {
  [-2]=>
  bool(true)
  [-1]=>
  bool(true)
  [0]=>
  bool(true)
  [1]=>
  bool(true)
}
array(4) {
  [-2]=>
  bool(true)
  [0]=>
  bool(true)
  [1]=>
  bool(true)
  [2]=>
  bool(true)
}