After logout I'm still able to view previous page in PHP. Do I need to do something or it's just ok as it is browser cached page. In logout page I simply unset all session variables and destroy session.
If you wish to prevent the user from seeing previous activity after logout, you can check for session data at start of the page and if it is empty redirect the user to login/index page. That way even if the user clicks back button after logging out, he'll be redirected to the login page.
if(empty($_SESSION['id'])){header('Location:index.php');}