not sure about this, but the current behavior also could be explained.  it is always exposed the intern state..  (maybe mark it as final could reslove the confused, but bc break)

It should NOT be marked as final I think.
I noticed that strange behaviour during actual usage of SplObjectStorage with really deep objects. Using __debugInfo() to print only object ids is a bless.

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This happens with ArrayObject as well. The problem is that internal classes that are extended do not allow for overriding outputting of the internal state.

Able to replicate the problem with each PHP version since this functionality has been added...

https://3v4l.org/ddq2i

For those not wanting to click on an external link...

Script:
----------------
<?php

class Base extends ArrayObject{
    var $parent = true;
    var $another_prop = 123;
    public function __construct() {
        $this['test'] = 123;
    }
    public function __debugInfo() {
        return [
            'parent'=>$this->getArrayCopy()
        ];
    }
}

class Child extends Base {
    var $parent = false;
}

var_dump(new Child());

Expected result:
----------------
object(Child)#1 (1) {
  ["parent"]=>
  array(1) {
    ["test"]=>
    int(123)
  }
}
Actual result:
----------------
object(Child)#1 (3) {
  ["parent"]=>
  bool(false)
  ["another_prop"]=>
  int(123)
  ["storage":"ArrayObject":private]=>
  array(1) {
    ["test"]=>
    int(123)
  }
}

Here's a simple case to illustrate the problem.
I create a class 'Mylist' that inherits from SplDoublyLinkedList.
Mylist has one more public property "type" (integer).
When var_dump i want to match the value to a name (from an array).
so :
final class Mylist extends \SplDoublyLinkedList
{
    /* @var int */
    public $type = 0;

    public function __debugInfo():array
    {
        return ['type' => Y::getName($this->type), "dllist" => $this->dllist];
    }
}
When using var_dump, the value for type should be a string returned by Y::getName and NOT the actual 'type' (integer) value.

BUT you can put whatever you want in '__debugInfo' it is currently TOTALY IGNORED.

(PHP 7.2.2 (cli) (built: Jan 31 2018 19:31:17) ( ZTS MSVC15 (Visual C++ 2017) x64 ))