PHP Fatal error: Class ‘DOMDocument’ not found
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.

THANK YOU!!!
John — June 20, 2008 @ 11:47 am
Thanks! First result in google & right on the money.
Sean — July 7, 2008 @ 8:53 pm
Thank you so much! I searched for hours for a fix and reinstalled it twice before I came along your site! You Sir, should get a medal!
Sweet — July 8, 2008 @ 9:22 pm
Roxxorin’ - first google click solves the problem.
Matt — July 18, 2008 @ 10:17 pm