Help:Adding parser functions
Adding parserfunctions - documentation at meta.wikimedia.org
Much more detailed info at meta.wikimedia.org, but basic info here:
Installation
Download all 4 of these files and put them in a new directory called ParserFunctions in your extensions directory.
If you don't have php5 use these files instead (older revision):
Then put the following at the end of your LocalSettings.php:
require_once( "$IP/extensions/ParserFunctions/ParserFunctions.php" );
If you get errors such as "Warning: require_once(/extensions/ParserFunctions/ParserFunctions.php) [function.require-once]: failed to open stream:", you should substitute the line for:
require_once( 'extensions/ParserFunctions/ParserFunctions.php' );