注意:PHP 会话的未定义索引

我有我的索引.php页面，用户可以在其中注册和登录会话。

这是我得到的错误：

注意：未定义的索引：用户名在 C：''wamp''www''CMS''admin''index.php 第 18 行。

我的代码有什么问题？

index.php的简短片段：

源代码：

<?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>

这是我的登录名.php

源代码：

<?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");
        }
    }
?>

if (session_status !== PHP_SESSION_ACTIVE) {
    session_start();
}

var_dump($_SESSION);