Parsing XML with SimpleXML

I'm attempting to use php and SimpleXML to assign variables from an XML output. This works:

$xml = simplexml_load_file($request_url) or die("feed not loading");

and when I do a:

echo "<pre>";
var_dump($xml);
echo "</pre>";

I get one of these two results depending on value of "Type" attribute.


object(SimpleXMLElement)#1 (2) {
["@attributes"]=>
array(1) {
["Type"]=>
string(5) "Popup"
}
["ad"]=>
object(SimpleXMLElement)#2 (6) {
["url"]=>
object(SimpleXMLElement)#3 (0) {
}
["cpvrate"]=>
string(5) "0.005"
["creativeid"]=>
string(10) "0204499385"
["campaignid"]=>
string(12) "2-0210728303"
["max-imp-per-user"]=>
string(1) "1"
["frequency-period"]=>
string(2) "24"
}
}


object(SimpleXMLElement)#1 (1) {
["@attributes"]=>
array(1) {
["Type"]=>
string(7) "NoPopup"
}
}


In my php code I'm assigning variables like this:

$adtype = (string) $xml->Type;
$outurl = (string) $xml->ad->url;
$cpvr = (string) $xml->ad->cpv_rate;
$crid = (string) $xml->ad->creativeid;
$caid = (string) $xml->ad->campaignid;
$mipu = (string) $xml->ad->max-imp-per-user;
$fp = (string) $xml->ad->frequency-period;

Only $outurl gets assigned. The other variables remain "empty". What should the correct syntax be for the other variable assignments?