Where is the $DOCUMENT_ROOT variable set?

Question

PHP

Good afternoon.
Actually where is the $DOCUMENT_ROOT variable set?
No database connection after site migration.
Connecting to the database is written like this:
include_once "$DOCUMENT_ROOT/auth.php";

Answer 1

I took it from the old server and re-uploaded php.ini to which I transfer and voila, it worked.
Compared these php.ini with diff
Here's the difference:
https://docs.google.com/document/d/1qi8OqeKvWerbr_...

Answer 2

This is old code. On new php you have something like $_SERVER['DOCUMENT_ROOT'] - php.net/manual/en/security.globals.php

Answer 3

And it is better to abandon DOCUMENT_ROOT altogether and address entry points relative to __FILE__, for example.
Where is the guarantee that no one will want to put your project in some subfolder of the virtual host and call it from there?