PHP - 似乎无法重定向我的PHP文件链接到另一个文件夹中的另一个文件

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:

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

To include server.php in your register page you need to use require('../server.php'). The two dots is a shortcut/alias for the parent directory.

if your index.php and server.php in inside same folder then you should use
include 'server.php'
else first specify the directory for example if server.php is inside /home/homeFirst/server.php then do this
include '/home/homeFirst/server.php';