PHP - Trouble Understanding $_session

Back with more noob questions

so I was following a page in setting up a login system and so far it works how I want except for one minor thing

So I want when a user is logged into the system to see Profile div instead of Login and Register and I do that in my index.php with the following

 <?php  if(isset($_SESSION['username'])){
	echo 
    	"<div class='profile'> 
  		    <a href='logout.php'>Logout</a>
  		    
  		    </div> 
  		    ";
    }
    else{
    	echo 
    	"<div class='loginregister'> 
  		    <a href='login.php'>Login</a>
  		    <a href='register.php'>Register</a>
  		    </div> 
  		    ";
    }
    ?>

So when a user logs in it hits the login.php which looks like this 

<?php
    require('dbconnect.php');

    session_start();
    // When form submitted, check and create user session.
    if (isset($_POST['username'])) {
        $username = stripslashes($_REQUEST['username']);    // removes backslashes
        $username = mysqli_real_escape_string($conn, $username);
        $password = stripslashes($_REQUEST['password']);
        $password = mysqli_real_escape_string($conn, $password);
        // Check user is exist in the database
        $query    = "SELECT * FROM `Users` WHERE User='$username'
                     AND password='" . md5($password) . "'";
        $result = mysqli_query($conn, $query) or die(mysql_error());
        $rows = mysqli_num_rows($result);
        if ($rows == 1) {
            $_SESSION['username'] = $username;
            // Redirect to user dashboard page
            header("Location: index.php");
        } else {
            echo "<div class='form'>
                  <h3>Incorrect Username/password.</h3><br/>
                  <p class='link'>Click here to <a href='login.php'>Login</a> again.</p>
                  </div>";
        }
    } else {

and it starts the session_start() function whilst also allocating $_SESSION['username'] = $username so in theory when index.php loads if SHOULD (?) hit the correct if statement in the index.php file now outputting the Profile div instead of the Login and Register div.

Except it doesn't.

However when I include the auth_session.php at the top of my index.php file like (currently uncommented to test)

//<?php //include auth_session.php file on all user panel pages 
//include("auth_session.php");
//?>

with the Auth_session file looking like

<?php
    session_start();
    if(!isset($_SESSION["username"])) {
        header("Location: login.php");
        exit();
    }
    else { 
    }
?>

It does show the correct Profile div instead of the Login  and Register div.

So I'm trying to understand what is happening here as from the look of it the $_SESSION['username'] is allocated within login.php and the session_start() function is also started in login.php

So why would I need auth_session.php to be ran in order for the correct divs to show (I've not included auth_session.php as I want people to be able to see the site that aren't logged in)

Warm regards

First let me explain my code.
It comprises of three php files.

 project_status.php:

<?php
 include 'inc_fn_header_and_menu.php';
function includeFile($file,$variable) {
    $var = $variable;
    include($file);
}
if (isset($_GET['id']) && $_GET['id']!="") {
    $pid    = $_GET['id'];
    $_SESSION['pidForApproval'] = $_GET['id'];
    $query = 'SELECT * FROM `profile` WHERE pid ='.'\''.$pid.'\'';
    $result=mysqli_query($db,$queryToRetrievePP) or die("There are no records to display ... \n" . mysqli_error());
    foreach ($result as $row) {
        $status = $row['status'];
    }        
}
...........some PHP and HTML code.......
	<div id="customerPurchaseApprovalForm">
<?php  
  echo '<p>APPROVAL FOR CUSTOMER PURCHASE</p>';
  $discountApprovalStatus = "Granted";
  if ($discountApprovalStatus == "Granted") {  
 
  includeFile("project_status_app.php",$highestannualvalue);
  } else {
     //.........
  }

In project_status_app.php I am attempting to retrieve pidForApproval from the $_SESSION array.

<?php
        // put your code here  UPDATE `pp` SET `customer_purchase_remarks` = 'hahaha' WHERE `pp`.`id` = 207;
        
        if ($_SERVER['REQUEST_METHOD'] == 'POST') {
            include '../../inc/fastlogin.php';      
            $sql = "UPDATE pp SET customer_purchase_remarks ='{$_POST['remarkstxt']}' WHERE pp.pid='{$_SESSION['pidForApproval']}'"; 
            $result = mysqli_query ( $fastdb, $sql ) ;
            if (mysqli_affected_rows($fastdb) != 1) { 
               $_SESSION['err_cpa_rmks'] = "<p>Error while updating WHERE id='{$_SESSION['pidForApproval']}'</p>";
                //echo "<p>Error while updating WHERE id='{$_POST['pidForApproval']}'</p>".mysqli_error($fastdb);                 
            } else { 
                $_SESSION['suc_cpa_rmks'] = "<p>Records was updated successfully.</p>";
                //echo "Records was updated successfully."; 
            }
            header ("location: project_status.php?id="$_SESSION['pidForApproval']);
            exit();
        }    
        
        ?>

When I load  project_status.php,  project_status_app.php is supposed to display the form. Once the user fills in the form the and the submit button has been pressed, the UPDATE statement is supposed to run and then it is supposed to navigate back to project_status.php?id=FA142. But the update is failing and the when the project_status.php is loaded back, the url looks like this http://localhost/fast/project_status.php?id= . The id is empty. It is supposed to be something like this http://localhost/fast/project_status.php?id=FA142. With the id being populated at the 

header ("location: project_status.php?id=".$_SESSION['pidForApproval']);

I suspected that my $_SESSION['pidForApproval'] is not being populated in project_status.php but I echoed back $_SESSION['pidForApproval'] in that file itself and I can see it is being populated. Hence, I suspect that the $_SESSION['pidForApproval'] is not being passed to project_status_app.php. I have already attempted to include session_start(); clause in project_status_app.php but that gives an error, stating that the session has already started, in inc_fn_header_and_menu.php. Can someone help me as to why the $_SESSION['pidForApproval'] is not being passed on to the project_status_app.php file. Thank you.