Warning: include(../../inc/functions.php): failed to open stream: no such file or directory

Warning: include(../../inc/functions.php): failed to open stream: no such
file or directory

This is the situation that I face off, the following is the structure of
the site:
[root]
index.php
[inc] // inside of this folder is functions.php
[css]
[img]
[connect] // inside of this folder is db.php and setup.php
[admin]
index.php
[img]
[css]
**[inc] // inside of this folder is functions.php**
[connect] // inside of this folder is db.php and setup.php to connect
the admin
[content] // here is page.php
[op] // here are add.php, modify.php, delete.php
I've tested using absolute and relative paths, DIR,
$_SERVER['DOCUMENT_ROOT'], etc
the problem is when I tried to include the functions.php file from the
folder inc to the folder content or op. I've tested in some different ways
and checks some answers from here I cannot fixed it.
Could someone fix this issue?