Description:
------------
We're currently upgrading to PHP7, and noticed a notice which wasn't present in PHP5.

In PHP5 there is a warning regarding modification of the array, due to the reference to $sortingData. In PHP7 however, an offset is missing. Strangely, when var_dump'ing the indexes of the array, the 0 index is present.

Test script:
---------------
<?php

ini_set('display_errors', true);
error_reporting(E_ALL);

$sortingData = [
  0 => 10,
  1 => 12,
  // This key is magic - anything >= 8 (or a non-integer key) makes this script produce the E_NOTICE
  8 => 11,
];

uksort($sortingData, function ($a, $b) use (&$sortingData) {
  return $sortingData[$a] > $sortingData[$b] ? -1 : 1;
});

var_dump($sortingData);


Expected result:
----------------
Warning: uksort(): Array was modified by the user comparison function in /home/richard/break.php on line 15
array(3) {
  [1]=>
  int(12)
  [8]=>
  int(11)
  [0]=>
  int(10)
}

Actual result:
--------------
Notice: Undefined offset: 0 in /home/richard/break.php on line 14
array(3) {
  [1]=>
  int(12)
  [8]=>
  int(11)
  [0]=>
  int(10)
}