Description:
------------
The function serialize() doesn't work with a DOMDocument object. After you've serialized and unserialized an object of DOMDocument the unserialized object seems to be empty (The object knows that it is a DOMDocument Object, but all data is lost).

Reproduce code:
---------------
<?PHP
	//----------------------------------------------Building a working DOMDocument-----------------------------------
	//create new DOMDocument
	$domDoc1 = new DOMDocument();
	//create some elements
	$rootElement = $domDoc1->createElement("Element1");
	$element2 = $domDoc1->createElement("Element2", "Value2");
	//adding element to root
	$rootElement->appendChild($element2);
	//adding root to DOM
	$domDoc1->appendChild($rootElement);
	//show that DOMDocument is working right now
	$elements1 = $domDoc1->getElementsByTagName("Element2");
	foreach ($elements1 as $val) {
	        echo "TagName = '".$val->tagName."' ";
			echo "Value = '".$val->nodeValue."'<br>";
	}
	//----------------------------------------------now the strange behavior-----------------------------------------
	//now serialize DOMObject
	$serialize1 = serialize($domDoc1);
	//The serialized DOMObject seems to be empty
	echo $serialize1."<br>";
	//unserialize 
	$unserialize1 = unserialize($serialize1);
	//----------------------------------------------the error--------------------------------------------------------
	//And now there is the error.. $unserialize1 seems to be no DOMDocument or an empty one
	$elements2 = $unserialize1->getElementsByTagName("Element2");
	//You get the error message: Warning: Couldn't fetch DOMDocument in ....
	//print_r shows an empty object
	print_r($unserialize1);
?>

Expected result:
----------------
I thought, when you unserialize a DOMDocument object you get a DOMDocument object with all data of the original one (NodeLists etc). 

Actual result:
--------------
After unserializing a serialized DOMDocument object you get an empty DOMDocument object.