Description:
------------
Found at https://twitter.com/kozzi11/status/806828728890380288

https://3v4l.org/suHQh

Removing the settype() call produces the expected output.

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

$ao = new ArrayObject;

foreach ([1, 2, 3] as $i => $var)
{
    settype($var, 'string');
    $ao[$i] = $var;
}

var_dump($ao);

Expected result:
----------------
Output for 5.6.0 - 5.6.28, hhvm-3.12.0 - 3.13.2, 7.0.0 - 7.0.14

object(ArrayObject)#1 (1) {
  ["storage":"ArrayObject":private]=>
  array(3) {
    [0]=>
    string(1) "1"
    [1]=>
    string(1) "2"
    [2]=>
    string(1) "3"
  }
}

Actual result:
--------------
Output for 7.1.0

object(ArrayObject)#1 (1) {
  ["storage":"ArrayObject":private]=>
  array(3) {
    [0]=>
    &string(1) "3"
    [1]=>
    &string(1) "3"
    [2]=>
    &string(1) "3"
  }
}