Why It is not possible to serialize PHP built-in objects?

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I have tried to unserialize a PHP Object.Warning: unserialize() [function.unserialize]: Node no longer exists in /var/www/app.php on line 42But why was that happen?Even if I found a solution to unserialize simplexml objects, its good to know why php cant unserialize objects?To serialize simplexml object i use this function\[code\]function serializeSimpleXML(SimpleXMLElement $xmlObj) { return serialize($xmlObj->asXML());}\[/code\]To unserialize an simplexml objetc i use this function\[code\]function unserializeSimpleXML($str) { return simplexml_load_string(unserialize($str));}\[/code\]