PHP AltoRouter : $_SERVER['BASE-URI'] returns NULL on my Raspberry Pi web server

Question

I worked on a personal project on my computer, and it works perfectly fine on the localhost of this computer. But I tried to put it online using my Raspberry Pi as a web server and it doesn't work (it shows a 404 error). It's a project using PHP AltoRouter. And I noticed that on my computer, $_SERVER['BASE-URI'] returns the url coming after 'localhost', but on my RaspberryPi, it returns 'NULL'.

In the public folder, my index.php file starts like this:

 <?php 
    require '../vendor/autoload.php';
    require '../app/Controllers/MainController.php';
    require '../app/Controllers/LevelController.php';

    $pageToDisplay = '/';
    // thanks to the htaccess redirection, no querystring can be seen in the url. Htaccess will add the information about the requested page
    if (isset($_GET['_url'])) {
        $pageToDisplay = $_GET['_url'];
    }

    $router = new AltoRouter();
    // we indicate to AltoRouter where the root of the site is with basePath
    $router->setBasePath($_SERVER['BASE_URI']);

My Htaccess file is like this:

RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule . index.php [L]

Has anyone any idea where this issue could be coming from ? When I set up my Rpi as a web server, I installed Apache2, PHP, MySQL, PHPMyAdmin, Git, VS Code, Composer (and I also wrote the commands 'composer install' and 'composer dump-autoload'). Is there something I forgot to do when I set up my Raspberry Pi ?

Thank you for your time !