Check that you're not using the wrong $IP in your include path, such as a path pointing to old versions of the files. Using a pathname with the version in it is suspicious when you say you're "extracting over the old files".
oh, yeah :( that was the problem, I had:
$IP = "/var/www/html/mediawiki-1.4.4";
and I lost a couple of hours trying to understand why the math part didn't work either until I realized of that!
Sorry for this but is good information for other newbies like me.
If you're using a PHP opcode cache system, try clearing it manually and/or editing the file to update its timestamp, to ensure an obsolete version of the User class definition isn't being used.
Thanks!, I have to learn that too,
Pablo
-- brion vibber (brion @ pobox.com)