MessagesXx.php

The MessagesXx.php files (for example, languages/messages/MessagesEn.php for English) used to contain both the language's localized system messages, as described in detail in Manual:MediaWiki architecture#Localizing messages, as well as other language-specific configuration.

In April 2014, as a result of this RFC, the localization messages (the $messages array) were moved to JSON files, leaving the PHP files only for the remaining language-specific configuration.