If you need to parse the data from SimpleXML into a session variable remember to define the data as a string first.
If you don't you will get warnings of "Node no longer exists" pointing to your session_start() function.
This will work:
<?php
$new_version = simplexml_load_file('http://example.com/version.xml');
$_SESSION['current_version'] = (string)$new_version->version;
?>