ログイン システムを使用して会員専用ページを保護するにはどうすればよいですか?

<?php
// Database connection
$servername = "localhost";
$username = "username";
$password = "password";
$db = "database";

// Create connection
$conn = new mysqli($servername, $username, $password, $db);

// Check connection
if ($conn->connect_error) {
  die("Connection failed: " . $conn->connect_error);
}

// Load custom functions
require_once('fn/functions.php');

<?php
require_once('inc/head.inc.php');
require_once('fn/init.php');
?>

<div>

<?php
$username = $_POST['username'];
$password = $_POST['password'];

// Validate credentials against database
if (authenticate($username, $password) == true) {
  // Set session variables
  session_start();
  $_SESSION['username'] = $username;
  
  echo 1; // Success
} else {
  echo 'Invalid credentials.';
}

<?php
require_once('inc/head.inc.php');
require_once('fn/init.php');

// Check if user is logged in
session_start();
if (!isset($_SESSION['username'])) {
  header('Location: index.php');
  exit;
}
%>

<h1>Welcome to the Restricted Page, <?php echo $_SESSION['username']; ?>!</h1>