- project
 - app
  - Config
   - file1.php
 - file.txt

require_once '../../file.txt';

$ ls -ld /var/www/html/project /var/www/html/project/app
         /var/www/html/project/app/Config /var/www/html/project/file.txt
drwxr-xr-x  6 root root 4096 May 30 11:01 /var/www/html/project
drwxrwxr-x 13 root root 4096 May 29 08:09 /var/www/html/project/app
drwxrwxr-x  3 root root 4096 May 30 05:29 /var/www/html/project/app/Config
-rw-r--r--  1 root root 3401 May 30 11:13 /var/www/html/project/file.txt