Codeigniter with several webs

I am not familiar with AWS Lightsale but do use an Apache2 VPS.

I have use common directory /var/www/ci2/  which contains the latest couple of CI Versions (and a few other common librarie).

Each CI domain has an index.php system link to the CI version system path.

I have set an index.php constant to the CI4 system path and set it in the App/Config/setup.php (I think from memory).

Tentatively tapped on a tablet

Can you please put the example of the index.php and setup.php files?