定义一个始终返回特定文件夹的常量

I am using a self-made framework with various directories, and I would like to create a constant which basically outputs the directory where my application folder is situated. I could accomplish this by declaring

define("DOC_ROOT", "/var/www/html/public/my-site.tld/");

somewhere in my public/index.php file, but for some reason, I would like to use a combination of __DIR__, realpath(), dirname()... as I still don't know why it is not working for me.

For now, let's assume this is a the folder structure I am using.

my-site.tld/
  src/
    public/
       index.php
    app/
       configs/
          constants.php/
       views/
       controllers/
       bootstrap.php

Since the bootstrap.php requires the constants.php first, and then index.php loads the bootstrap.php as shown

// public/index.php
require '../boostrap.php`; 

We can assume this constants are the first to run, within the application. Now, the problem is that declaring a doc root which outputs just the full path to my-site.tld or in other words

/var/www/html/public/my-site.tld/

has become impossible to me, partly because echoing DOC_ROOT would give out different paths in different files. So, I am asking is there a way to just define DOC_ROOT in the constants file, and wherever it is called, it will always output the main application folder as opposed to doing it manually like

define("DOC_ROOT", "/var/www/html/public/my-site.tld/");

Which require me to manually change that path everything I make changes or rename my folders.

Store the URL in a standard constant:

define("CONSTANTNAME",     "VALUE");

PHP Constant Documentation