PHP sessions

I have a problem with a login on my site. The script starts session and login works, sets session id and i can return that and echo it in variable, but when i navigate to a different file on my server, even one in the same directory, it says i'm not logged in.

login script

PHP Code:

---

<?php 
ini_set("display_errors", "1");
error_reporting(E_ALL);
session_start();

include_once "scripts/connect_to_mysql.php";

if (isset($_POST['username'])){

    $username = $_POST['username'];
    $hash = sha1($_POST['password']);

    $sql = "SELECT * FROM members WHERE username= '".$username."' AND password='".$hash."' LIMIT 1";
    $res = mysql_query($sql);
    if(mysql_num_rows($res) == 1){
        $row = mysql_fetch_assoc(mysql_query("$sql"));
            if($row['id'] > 0){
            $_SESSION['id'] = $row['id'];
            $_SESSION['username']= $row['username'];
            $_SESSION['email']= $row['email'];
            $id = ($_SESSION['id']);
            $username = ($_SESSION['username']);
            $email = ($_SESSION['email']);
            
            }
        echo "<font color='green'>Login successful.</font>";
        echo $id;
        exit();
    }
    else{
        echo "<font color='red'>Login not successful. Please try again.</font>";
        exit();
    }
}

?>

---

and i can put this on any other page and it says im not logged in

PHP Code:

---

<?php
ini_set("display_errors", "1");
error_reporting(E_ALL);
if(!isset($_SESSION['id']) || empty($_SESSION['id'])) {
    print 'You are not logged in';
    die();
    }
if(isset($_SESSION['username'])){
    print 'LOGGED IN!!!';
    }
?>

---

No errors are returned.

If anyone has any idea for me it is much appreciated.

to use session you must start it in every file where the session is used, or you can simply make a library file containing common used stuff and include it in a the pages.
so add sesstion_start(); in the second file