Notice: Undefined index with PHP session - php

Question

I have my index.php page, which is where users can register and log in with sessions.
This is the error I'm getting:
Notice: Undefined index: username in C:\wamp\www\CMS\admin\index.php on line 18.
What's wrong with my code?
Short snippet of index.php:
Source code:
<?php include "includes/admin_header.php" ?>
<div id="wrapper">
<!-- Navigation -->
<?php include "includes/admin_navigation.php" ?>
<div id="page-wrapper">
<div class="container-fluid">
<!-- Page Heading -->
<div class="row">
<div class="col-lg-12">
<h1 class="page-header">
Welcome To Admin
<small><?php echo $_SESSION['username'] ?></small>
</h1>
</div>
</div>
This is my login.php
Source code:
<?php include "db.php"; ?>
<?php session_start(); ?>
<?php
if (isset($_POST['login'])) {
$username = $_POST['username'];
$password = $_POST['password'];
// For login security
$username = mysqli_real_escape_string($connection, $username);
$password = mysqli_real_escape_string($connection, $password);
$query = "SELECT * FROM users WHERE username = '{$username}' ";
$select_user_query = mysqli_query($connection, $query);
if(!$select_user_query){
die("QUERY FAILED: " . mysqli_error($connection));
}
while($row = mysqli_fetch_array($select_user_query)) {
$db_user_id = $row['user_id'];
$db_username = $row['username'];
$db_user_password = $row['user_password'];
$db_user_firstname = $row['user_firstname'];
$db_user_lastname = $row['user_lastname'];
$db_user_role = $row['user_role'];
}
if ($username !== $db_username && $password !== $db_user_password) {
header("Location: ../index.php");
} else if($username === $db_username &&
$password === $db_user_password) {
$_SESSION['username'] = $db_username;
$_SESSION['firstname'] = $db_user_firstname;
$_SESSION['lastname'] = $db_user_lastname;
$_SESSION['user_role'] = $db_user_role;
header("Location: ../admin");
}
else {
header("Location: ../index.php");
}
}
?>

Answer 1

On the first line of your PHP script, write this:
if (session_status !== PHP_SESSION_ACTIVE) {
session_start();
}
For debugging purposes, add
var_dump($_SESSION);
after the if { } block. You also have to be sure that the session exists in every file you want to use it. To simplify the process I'll give you a short example:
You create a header.php file where you put the if {} block from above.
Whenever you want to use the session in a script, you just include that header file on the first line of your script.
//header.php
if (session_status !== PHP_SESSION_ACTIVE) {
session_start();
}
//index.php
include 'header.php';
echo $_SESSION['username'];
//login.php
include 'header.php';
$_SESSION['username'] = 'John Doe';