ป้องกันการ back กลับ หลังจาก login, logout ออกไปแล้ว ทำไมยัง back กลับปกติได้ครับ หรือทำผิดพลาด ชี้แนะด้วยครับ

<?php
    
    require_once('database/connection.php'); 
    
    if (isset($_POST['submit'])) { 

        $username =  $conn->real_escape_string($_POST['username']);
        $password = $conn->real_escape_string($_POST['password']);
        date_default_timezone_set('Asia/Bangkok');
        $Newtime = date('Y-m-d H:i:s');
        $Newtime1 = date('Y-m-d H:i:s');

        $sql = "SELECT * FROM `user` WHERE `username` = '".$username."'";
        $result = $conn->query($sql);
        $row = $result->fetch_assoc();

        if(!empty($row)){
            if(password_verify($password, $row['password'])){
                $update = "UPDATE `user` SET updated_at = '$Newtime', updated_dt = '$Newtime1'
                 WHERE `username` = '$username'";
                $runupdate = $conn->query($update);

                $_SESSION['id'] = $row['id'];
                $_SESSION['firstname'] = $row['firstname'];
                $_SESSION['lastname'] = $row['lastname'];
                $_SESSION['username'] = $row['username'];
                $_SESSION['password'] = $row['password'];
                $_SESSION['image'] = $row['image'];
                $_SESSION['created_at'] = $row['created_at'];
                $_SESSION['updated_at'] = $Newtime;
                $_SESSION['updated_dt'] = $Newtime1;
                $_SESSION['status'] = $row['status'];

                session_write_close();
               
                 if ($_SESSION['status'] == 'admin') {
                    echo "<script>alert('เข้าสู่ระบบสำเร็จ...'); window.location ='../cmssystem/pages/admin/dashboard/';</script>";
                }
    
                if ($_SESSION['status'] == 'member') {
                    echo "<script>alert('เข้าสู่ระบบสำเร็จ...'); window.location ='../cmssystem/pages/users/dashboard/';</script>";
                    
                }

            } else {
                echo '<script> alert("username หรือ password ไม่ถูกต้อง") </script>';
                header('Refresh:0; url=index.php');
            }
        }else{

            echo '<script> alert("ไม่สามารถเข้าสู่ระบบได้โปรดกรอกข้อมูลใหม่อีกครั้ง")</script>';
            header("Refresh:0");
        } 
    }
?>

<?php
    require_once('../../../database/connection.php');
    if($_SESSION['id'] == "")
    {
        echo "Please Login!";
        exit();
    }

    if($_SESSION['status'] != "admin")
    {
        echo "This page for Admin only!";
        exit();
    }   
    
    $sql = "SELECT * FROM user WHERE id = '".$_SESSION['id']."' ";
    $query = $conn->query($sql);
    $result = $query->fetch_assoc();
?>

<?php 
   
    session_start(); 
    session_destroy(); 

    echo "<script>alert('ยืนยันการออกจากระบบ...'); window.location ='../../index.php';</script>";
?>