MessagesXx.php
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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.