If the user is not logged in, check the cookie/session and redirect to the login page.

If login is not mandatory but you want to ensure the user sees the login page, create a session that is started in the login page. And check if the session exists on other pages and then redirect

First, if your web application allows user authentication, do so.

On a page you don't want unauthenticated users to access, add this code to the top of your code script, for example profile.php

session_start();
if(!isset($_SESSION){
   header("location:login.php");
}

Then, add this code snippet on the login page.

session_start();
if(isset($_SESSION){
header("location:profile.php");
}