Description:
------------
I don't know if it's bug but the ArrayObject allows foreach by reference. 

If however I create it with a custom iterator which overrides the current() method a fatal error is produced: 

Fatal error: An iterator cannot be used with foreach by reference

Links with both cases can be found here:
https://3v4l.org/aBKtd
https://3v4l.org/00Z3F


Test script:
---------------
$a = new ArrayObject([1, 2, 3]);
foreach ($a as $key => &$value) {
    $value += 10;
}
unset($value);

var_dump($a);

Expected result:
----------------
Fatal error: An iterator cannot be used with foreach by reference

Actual result:
--------------
object(ArrayObject)#1 (1) {
  ["storage":"ArrayObject":private]=>
  array(3) {
    [0]=>
    int(11)
    [1]=>
    int(12)
    [2]=>
    int(13)
  }
}