Description:
------------
The reference to the UnSerializable object is serialized as a null, instead of as a reference. In addition, the serializer still sees it as a reference, causing the count of reference-able objects to be off. In my test script, $recovered[3] references $recovered[1] instead of $recovered[2].

This might be related to https://bugs.php.net/bug.php?id=77302, which also involved an empty serialize() method, and was fixed in the same version this was introduced, namely 7.3.4.

Version comparison: https://3v4l.org/YD9fS

Test script:
---------------
class UnSerializable implements Serializable
{
  public function serialize() {}
  public function unserialize($serialized) {}
}

$unser = new UnSerializable();
$arr = [$unser];
$arr[] = &$arr[0];
$arr[] = 'endcap';
$arr[] = &$arr[2];

$data = serialize($arr);
echo $data . PHP_EOL;
$recovered = unserialize($data);
var_export($recovered);

Expected result:
----------------
a:4:{i:0;N;i:1;R:2;i:2;s:6:"endcap";i:3;R:3;}
array (
  0 => NULL,
  1 => NULL,
  2 => 'endcap',
  3 => 'endcap',
)

Actual result:
--------------
a:4:{i:0;N;i:1;N;i:2;s:6:"endcap";i:3;R:3;}
array (
  0 => NULL,
  1 => NULL,
  2 => 'endcap',
  3 => NULL,
)