Description:
------------
I believe that the way serialization works for objects 
doesn't make sense.  I have a subclass whose superclass 
contains a private member variable.  Upon serialization, I 
cannot get the private member variable to serialize for the 
subclass (except by using a "NUL class-name NUL member-name" 
string in __sleep()).

Imho, subclasses shouldn't have to know what parts of their 
parent classes to serialize.  I can think of two possible 
solutions to this problem:

1. Have serialize() walk the inheritance tree for the object 
it is serializing.

2. Have some method that will be able to take the output 
from parent::__sleep() and modify it so that it can be 
passed back from the __sleep() method of the subclass so 
that private member variables in the parent can be 
serialized.  (The function I have in mind would do the NUL 
class-name NUL member-name transformation).

Reproduce code:
---------------
class A {
  private $a;
  public function __sleep() { return array('a'); }
  public function getA() { return $this->a; }
  public function setA($a) { $this->a = $a; }
}

class B extends A {
}

$b = new B();
$b->setA(10);
$bSerialized = serialize($b);
$bUnserialized = unserialize($bSerialized);
var_dump($b);
var_dump($bUnserialized);

Expected result:
----------------
object(B)#1 (1) {
  ["a:private"]=>
  int(10)
}
object(B)#1 (1) {
  ["a:private"]=>
  int(10)
}

Actual result:
--------------
object(B)#1 (1) {
  ["a:private"]=>
  int(10)
}
object(B)#2 (2) {
  ["a:private"]=>
  NULL
  ["a"]=>
  NULL
}