The above error is appears to be a common occurrence when you upgrade your Mediawiki installation from version 1.11 to 1.12. To fix this you need to have your system administrator do the following:
yum -y install php-xml
service httpd restart
The installs the XML modules into PHP. Restarting httpd allows the web server, in this case Apache, to load the new modules.
This solution came from the Mediawiki Support Desk.
