Serializing SimpleXML objects?

From the following code snippet, it obviously is, but is it possible to use
them once they're unserialized?

$xml = '<entity1 name="fred">Some Text<entity2 name="bert" /></entity1>';
$sxml = simplexml_load_string($xml);

echo htmlspecialchars($sxml->asXML());

$ser_xml = serialize($sxml);

//Raises Warning: Node no longer exists in... (twice)
$unser_xml = unserialize($ser_xml);

//Raises Warning: Node no longer exists in... (twice)
echo htmlspecialchars($unser_xml->asXML());

This may seem a strange requirement, but I'm attempting to see if
deserializing has better performance than simplexml_load_file(). (I'm
cacheing portions of a potentially large xml file, so I don't have to load
the whole file for every request)

Regards

Philip