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 ( table) are actually valid. Existing usernames can become invalid if   is altered or if   is changed.

Usage
In case invalid usernames are found, the script returns the user_id and the username for each of the affected users.