Is there a way for PHP files to have an absolute-path to subfolders, rather than a relative-path with Apache without using "../assets/autoload.php"?

Good day, I am making a website. I'd like this site to have an account-system. Following multiple tutorials and teaching myself, I believe I've made a decent system using Apache and MySQL.

However, when linking the PHP files from the "assets/private" folder, it doesn't appear to work within subfolders. For example, this is my autoload.php file which runs the database functions;

<?php
session_start();
ini_set("display_errors",0);

require "../private/database.php";
require "../private/functions.php";
?>

And here is the code on the top of every HTML file to prevent those that are not logged in from viewing its pages;

<!php
    require '../private/autoload.php';
    $user_data = check_login($connection);
    
    $username = '';
    if(isset($_SESSION['username']))
    {
        $username = $_SESSION['username'];
    }
?>

All of that works perfectly! But as you can see, it works within one "folder path", with ../private/autoload.php. I need it to run within multiple folders, potentially folders of folders. Rather than have it relative, which appears to not be working, is there a way to create an absolute link?

require '/websitename/private/autoload.php';
$user_data = check_login($connection);

I have tried that example, which works for stylesheets, JavaScript files, images and videos, yet it does not work with PHP files for some reason.

These paths work;

/websitename/home/index.php - Home Page
/websitename/home/changelog.php - Changelog page

And the Library paths;

/websitename/library/index.php - Library page
/websitename/library/list.php - List page

But subfolders do no work;

/websitename/library/article/index.php - Library > Article page
/websitename/library/countries/index.php - Library > Country page
/websitename/library/countries/country/index.php - Library > Country > CountryName page

How would I apply an absolute path to fix the subfolder issue? Or is there another solution? I have looked this up on multiple sites and nothing appears to give an answer. I have seen one vague answer that apparently the .htaccess file may be require an option setting to tell each file where it's getting it's information from, since by "default", it's within one folder-path yet didn't explain which settings that would be.

Hopefully this all makes sense! First post so I hope I met the requirements <3

Solution

How would I apply an absolute path to fix the subfolder issue?

Unlike .html, .css, .js, images & video files, that have their paths within the main site folder, paths from php code within .php files are taken from the server's source. Which, for me using Apache results in;

require "C:/xampp/htdocs/website/database/autoload.php";

Thank you for your answers!