Storing SimpleXMLElement values in $_SESSION does not work. Saving the results as an object or individual elements of the object will result in the dreaded "Warning: session_start() [function.session-start]: Node no longer exists" error.
For example, this does not work:
$xml = new SimpleXMLElement($page);
$country = $xml->Response->Placemark->AddressDetails->Country->CountryNameCode;
$_SESSION['country'] = $country;
This will work:
$_SESSION['country'] = (string) $country;