trying to navigate to a page

hi everyone
In my index.php file I have a form used for logging in which has login.php as the action

in login.php I have

<?php


require_once 'cls_user.php';

if (isset($_POST['uid']) && isset($_POST['pwd']))
{
    $user = new user;
    $user->let_uid($_POST['uid']);
    $user->let_pwd($_POST['pwd']);
    if ($user->authenticate())
    {
        //echo "authenticated!";
        header('index.php');
    }
}



?>

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when the user class authenticates the login it starts a session variable, the idea being when   returning control back to index.php it sees the session variable has been set and acknowledges the user.

the problem is that the 'header' command isn't doing anything, I thought it would go back to index.php but instead I just get a blank screen (or just 'authenticated!' if I uncomment it)

how can I make it go back to index.php or what approach do I need to make it work