存储会话用户名，并回显它

Sorry for all the questions today, but I don't seem to be getting anywhere.

I have a login script as follows:

$sql = "SELECT * FROM users WHERE user_name='$user' AND password='$pass'";
$user_nickname = $row['user_nickname'];
$result = mysqli_query($con, $sql);
if($row = mysqli_fetch_array($result)) {
  $_SESSION['loggedIn'] = true;
  $_SESSION['isAdmin'] = ($row['is_admin'])? true : false;
  $_SESSION['userNickname'] = $user_nickname;
  header('Location: index.php');
}else{
  echo 'Incorrect User
}

Then on my index page, I pull files via AJAX into the centre, so for example on the user profile page I have

<h1 class="page-header">Hello, <?php session_start(); echo $_SESSION['userNickname']; ?></h1>

At the start of the page where this code is, I have:

<?php
session_start();
if( ! $_SESSION['loggedIn'] ) {
    // not logged in redirect direct to login page
    echo "<div id='loadcontent' class='col-sm-9 col-sm-offset-3 col-md-10 col-md-offset-2 main'>";  
    include "loginscreen.php";
    echo "</div>";
    die();
}
?>

I have also tried

if (isset($_SESSION["userNickname"])) {
echo $_SESSION["userNickname];
}

Am I missing something? I have been searching around, but I can't seem to understand why it's not being stored.

The code I am expecting is "Hello, NICKNAME"