php-PHP下xml无法正常取值

输入数据：
$xml=SimpleXMLElement Object
(
[URL] => http://test.net/index.php?m=home&c=index&a=test
[ToUserName] => touser
[FromUserName] => fromuser
[CreateTime] => 20160129
[MsgType] => text
[Content] => test_message
[MsgId] => 01
)
$t=$xml->FromUserName;
print_r($t);
结果是：
SimpleXMLElement Object
(
[0] => fromuser
)

取不到值。实在无解求帮助。