MediaWiki is an extremely powerful, scalable software and a feature-rich wiki implementation that uses PHP to process and display data stored in a database, such as MySQL. MediaWiki is geared towards the needs of the Wikimedia projects. The program was primarily developed to run on a large server farm for Wikipedia and its sister projects. Features, performance, configurability, ease-of-use, etc are often designed in this light; if your needs are radically different the software might not be appropriate for you.