Thanks for the question.
That is usually caused by server not having default document configured. In case of IIS/IIS express we should do it automatically.
For other server types you can either request a php file directly, e.g. index.php
- for Apache set it up in htaccess file
- for PHP build-in configure router script ( you can do it in PHP project properties, Server tab) ... More information on php.net
Please let me know if that helped.