How to detect the path to the application root?

auge

I’m trying to dynamically detect the root directory of my page in order to direct to a specific script.

echo ($_SERVER['DOCUMENT_ROOT']); 

It prints /myName/folder/index.php

I’d like to use in a html-file to enter a certain script like this:

<a href="<?php $_SERVER['DOCUMENT_ROOT'].'lib/logout.php'?>">log out</a>

This seems to be in bad syntax, the path is not successfully resolved.

What’s the proper approach to detect the path to logout.php?

The same question in different words:

How can I reliably achieve the path to the root directory (which contains my index.php) from ANY subdirectory? No matter if the html file is in /lib/subfolder or in /anotherDirectory, I want it to have a link directing to /lib/logout.php

On my machine it’s supposed to be http://localhost/myName/folder (which contains index.php and all subdirectories), on someone else’s it might be http://localhost/project

How can I detect the path to application root?