If you want to serialize and unserialize SimpleXMLElement objects for caching, you need to transform the SimpleXMLElement object into a standard class object before unserializing.
This is only if you want to cache converted data, the functionallity of the SimpleXMLElement will not be held.
$content = '<SomeXML....'
$serialized = str_replace(
array('O:16:"SimpleXMLElement":0:{}', 'O:16:"SimpleXMLElement":'),
array('s:0:"";', 'O:8:"stdClass":'),
serialize(simplexml_load_string($content))
);
$unserialized = unserialize($serialized);