Manual:$wgUsePHPTal

Details
PHPTal is a library for page templates. MediaWiki includes a recent PHPTal distribution. It is required to use the Monobook (default) skin on versions older than 1.4.0.

Currently it does not work on PHP5.

The default value for versions older than 1.4.0 is calculated automatically based on the current PHP version, but may be overridden.