Loading an application begins with a public/index.php file. It contains include startup(bootstrap) application file (default: /app/bootstrap/app.php), which returns an extended Slim3 class. To run the application should contain the launch line $app->run();. You can, at its discretion rewrite the contents of this file. It is better to to use another bootstrap file.

Copy the default bootstrap file app/bootstrap/app.php to new file in folder: app/bootstrap/.

Make new logic, or change default.

This file always return extended class Slim\App

Change default loaded file in public/index.php to new created file. (require_once '../app/bootstrap/custom_loader.php')