I'm having issues with a login form for my website. At the top of the login.php form I have this

<?php 
if (isset($_SESSION['username'])){
header("Location: http://myurl/"); die;}?>
<?php require 'includes/dbconnect.php' ; 
?> 
<?php require 'includes/header.php';  ?>

and when login is successful I redirect them to

header('Location: http://myrul/');

Header.php has

<?php session_start(); ?>

at the top.

When I tried to login, I get this error message:

Warning: Cannot modify header information - headers already sent by (output started at /path/to/file/login.php:7) in /path/to/file/login.php on line 39.

login.php:7 ==> <?php require 'includes/header.php'; ?>

while

Line 39 in login.php is ==> header('Location: http://myrul/');

Please where is the problem