Warning: Cannot modify header information about header

Question

I have some warning if login successful.

this is an error

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\PSI\index.php:25) in C:\xampp\htdocs\PSI\proseslogin.php on line 32

There are my sources.

proseslogin.php

if ($password == $data['password'])
{
    // menyimpan username dan level ke dalam session
    $_SESSION['user_akses'] = $data['user_akses'];
    $_SESSION['username'] = $data['username'];


    include "index.php";


}
else echo "<h1>Login Failure</h1>";
header("Refresh:2; url=login.php");
?>

This is an error

line 32 =>  header("Refresh:2; url=login.php");

That header for failure login which can refresh about 2 sec and go back to logion session in login.php

FYI : I use PHP 7

Answer 1

Everything is simple - you can't send headers AFTER you output anything on the client side (no matter if it's text, html or just a blank space). Try to use header before outputting the content, and you'll see your error will be gone.

Try this:

 if ($password == $data['password']) {
    // menyimpan username dan level ke dalam session
    $_SESSION['user_akses'] = $data['user_akses'];
    $_SESSION['username'] = $data['username'];


    include 'index.php';


}else { 
//headers before the content
header('Refresh:2; url=login.php');
//and then content here
echo '<h1>Login Failure</h1>';
}