PHP - Can't seem to redirect my php files to link to another file that is in another folder

i am a beginner in PHP and I am trying to learn how to make a simple registration form with mysql and php.

The problem is I cannot link to another file that is in another folder. Here are some pictures:

1. 

Even though i have placed this in my index.php codes and for other files.

<?php 
        session_start(); 

        if (!isset($_SESSION['username'])) {
            $_SESSION['msg'] = "You must log in first";
            header('location: pages/login.php');
            exit();
        }

        if (isset($_GET['logout'])) {
            session_destroy();
            unset($_SESSION['username']);

        }

    ?>

I have huge problems trying to understand how to identify the error when I include this line for a server connection in my register.php

<?php include('server.php') ?>

However it will only work if I place all the files outside of their own respective sub folders, even though I had placed the directory address. I don't understand, please help me understand what I am doing wrong.

Thank you