How can we include php files without specifying the subfolder path

Normally we use the following code to include php files inside each other:

<?php 

include_once 'include/config.php';
// OR
include 'include/config.php'; 
// OR 
include_once $_SERVER['DOCUMENT_ROOT'].'include/config.php';
// ect...
?>

But the above codes only apply if the php files are in the root file. I mean, if we move our files into a subfolder. We need to make a change to the code we included in the php files. For example like this:

<?php 
    include_once 'subfolder/include/config.php';
    // OR
    include 'subfolder/include/config.php'; 
    // OR 
    include_once $_SERVER['DOCUMENT_ROOT'].'/subfolder/include/config.php';
    // ect...
?>

What I'm saying is that when we move our php files into the subfolder, then include_once want to see subfolder name like (include_once 'subfolder/include/config.php';). This is a challenging situation because we need to do this for the included page in many files.

For example i include the include_once $_SERVER['DOCUMENT_ROOT'].'/functions/includes.php'; from the index.php and also included this includes.php file from all php files like header.php, posts.php, and ajax_post.php . It is working fine from the root folder but if we move the files in the subfolder then include.php file not including without subfolder name.

Maybe it is also possible to do this with .htaccess.

I have made this htaccess code maybe you have a solution with htaccess. I must be say i have tryed to use RewriteBase /subfoldername/ but include files didn't worked.

Options +FollowSymLinks -MultiViews
RewriteEngine On 
RewriteBase /

RewriteCond %{REQUEST_METHOD} !POST
RewriteCond %{THE_REQUEST} \s/+(.+?)\.php[\s?] [NC]
RewriteRule ^ /%1 [R=302,NE,L]

RewriteCond %{REQUEST_METHOD} !POST
RewriteCond %{THE_REQUEST} /index\.php [NC]
RewriteRule ^(.*)index\.php$ /$1 [L,R=302,NC,NE]

RewriteCond %{REQUEST_FILENAME} -d [OR]
RewriteCond %{REQUEST_FILENAME} -f
RewriteRule ^ - [L]

RewriteRule ^group/([\w-]+)/?$ sources/group.php?group_username=$1 [L,QSA]  
RewriteRule ^profile/([\w-]+)/?$ sources/user_profile.php?username=$1 [L,QSA]
RewriteRule ^profile/(followers|friends|photos|videos|locations|musics)/([\w-]+)/?$ sources/$1.php?username=$2 [L,QSA]     

RewriteRule ^admin/(.*)$ admin/index.php?page=$1 [L,QSA]

RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.+?)/?$ $1.php [L]  

RewriteRule ^(.+?)/?$ index.php?pages=$1 [L,QSA]

. My responsibility is, how can we include php files without subfolder name?

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