There is nothing in the xampp package (at least that I am aware of) that gives you something when you visit localhost/mysql.
That is to say, there is certainly nothing stopping you from creating your own mysql directory and installing your own php based software in that directory, but I don't know of any software packages that do specifically the thing you are speaking of. MySQL is a database software module and SQL language variant, not a PHP module, though there do exist commands in PHP with to interact with MySQL.
If you merely need to remotely administrate users, you can of course use a PHP-based CMS (Drupal, Joomla, etc.) system, a phpBB board, a MediaWiki, etc.
But I have never personally heard of or come across a php module solely for administrating users in a MYSQL database. Though phpmyadmin certianly can achieve this functionality. This does not mean that such at thing does not exist, though perhaps you might want to elaborate on what exactly it is you want to administer.