如何防止用户在未登录的情况下打开其他页面

Question

是接受用户名和密码的登录页面代码。但其他页面也是用URL打开的。使用的会话也开始。请帮我修改代码。

<?php
if(isset($_POST["submit"])){

if(!empty($_POST['user']) && !empty($_POST['pass'])) {
    $user=$_POST['user'];
    $pass=$_POST['pass'];

    $con=mysqli_connect('localhost','root','') or die(mysql_error());
    mysqli_select_db($con, 'user_registration') or die("cannot select DB");

    $query=mysqli_query($con, "SELECT * FROM login WHERE username='".$user."' AND password='".$pass."'");
    $numrows=mysqli_num_rows($query);
    if($numrows!=0)
    {
    while($row=mysqli_fetch_assoc($query))
    {
    $dbusername=$row['username'];
    $dbpassword=$row['password'];
    }

    if($user == $dbusername && $pass == $dbpassword)
    {
    session_start();
    $_SESSION['sess_user']=$user;

    /* Redirect browser */
    header("Location: index.php");
    }
    } else {
    echo "Invalid username or password!";
    }

} else {
    echo "All fields are required!";
}
}
?>