Description:
------------
$that is not available in parent class B (uncomment code in class A you will see it). So it is not possible to clone  class in a recursive way.



Reproduce code:
---------------
class A {
    public $a;
    function __clone(){
       //if( $that === null ) die('$that is null');
       $this->a = $that->a;
    }
}
class B extends A {
   public $b;   
   function __clone(){
      parent::__clone();
      $this->b = $that->b;
   }
}
$b = new B();
$b->a = 'AAA';
$b->b = 'BBB';
$clone =  $b->__clone();
print_r($clone);

Expected result:
----------------
B Object
(
    [b] => BBB
    [a] => AAA
)

Actual result:
--------------
B Object
(
    [b] => BBB
    [a] =>
)