unable to overload simplexml object?

[viveleroi0]

I have a system that loads in an xml file using simplexml and then attempts to append a new member variable. I've been using this same code for the past year on dozens of servers with no problems, but its not working here:

http://www.cogginsfulfillment.com/obj_test/object.php

PHP 5.1.2, no modifications.

Code:

$sxml = simplexml_load_file('register.xml');
$sxml->letsoverload = 'it worked';
print '<pre>';
print_r($sxml);
print '</pre>';