Manual:$wgTidyInternal/en

Details
Normally you shouldn't need to override the setting except for debugging.

Tidy 2.0 is bundled with PHP5 by default (MediaWiki requires PHP5), but is not included in the ubuntu build of PHP5. To install it on Ubuntu 10.04 / PHP 5.2.x:

Add $1 to $2; set $3 in $4; finally, restart apache.