分析SimpleXMLElement对象时会话不工作

Array
    (
        [0] => SimpleXMLElement Object
            (
                [userid] => 2
                [username] => UserName
                [userpassword] => 40bd001563085fc35165329ea1ff5c5ecbdbbeef
                [usertype] => A
                [createdBy] => 1
            )

    )

我想将userid存储在会话变量中

if(!empty($nodes))
{
  foreach($nodes as $node)
  {
   $UserId=$node->userid;
  }
}
$_SESSION['UserId1']= $UserId; 

Warning: session_start() [function.session-start]: Node no longer exists

而会议不是去拿。可以吗有人提出解决办法吗