Manual:CheckUsernames.php

Details
checkUsernames.php file is a maint-scripts>Special:MyLanguage/Manual:Maintenance scripts|maintenance script that is used to check that all usernames in the database (usertbl>Special:MyLanguage/user table|user table) are actually valid. Existing usernames can become invalid if   is altered or if chars>Special:MyLanguage/Manual:$wgMaxNameChars|$wgMaxNameChars is changed.