Description:
------------
When class extends with DOMDocument it's can't clone properly

Reproduce code:
---------------
<?php

class First extends DOMDocument {
	private $_first = null;

	public function setFirst($first) {
		$this->_first = $first;
	}

}

class Second extends First {
	public function __construct() {
		$this->setFirst('test');
	}
}

$obj = new Second;
$object = clone $obj;

var_dump($obj, $object);

?>

Expected result:
----------------
Object will be identical