Thread:Extension talk:Zend Search Lucene for MediaWiki/Fatal Error calling mb strtolower()/reply

AFAIK, "mb_strtolower" depends on an extra PHP library....

http://php.net/manual/en/mbstring.installation.php

Alternative you could try to substitute the function with "strtolower". I m afraid there are some more lines which uses this PHP library, so the savest way is to configure your PHP accordingly.

Keep in mind that this dependency is already mentioned in the requirements (mbstring).

cheers.