in /var/www/ is my main.php file:
<?php
include("../config/config.php");
header('Content-type: text/plain');
echo $user;
?>
in /var/config/ is my config.php file
<?php
$user = "myUser";
?>
When calling main.php in my browser I'm getting a blank site, instead of "myUser".
If your document root is set to "/var/www/" then you should work in that directory.
Your code does not work properly because of the "/", on Windows it doesn't matter but on Linux it does and it's different.
Anyway, a more better example would be this:
<?php
include("..".DIRECTORY_SEPARATOR."config".DIRECTORY_SEPARATOR."config.php");
header('Content-type: text/plain');
echo $user;
?>
And now main.php will be able to access your config.php on any operating system.