MediaWiki, the software used by Wikipedia, is written in PHP.
Wikipedia initially ran on UseModWiki, which was written in Perl, but that system soon struggled with the encyclopedia’s rapid growth. Developer Magnus Manske created a replacement in PHP, and it went live on Wikipedia on January 25, 2002. Lee Daniel Crocker later produced another major PHP-based rewrite.
PHP is the core server-side language, but MediaWiki is not limited to one technology. It uses databases such as MariaDB, PostgreSQL, or SQLite, while JavaScript powers many browser-side features. Thousands of extensions add capabilities such as visual editing, search, and multimedia handling.
A common mix-up is confusing MediaWiki with Wikipedia itself. Wikipedia is the encyclopedia and community; MediaWiki is the open-source wiki platform that runs it. The same software also powers projects including Wiktionary, Wikimedia Commons, Wikidata, and many independent websites.