PHP session side-effect warning - how to get solve?

I'm fairly new to php, and am sure this is easy, but I'd like to do it the right way. I have this script:

<?php
if ($_POST["username"]=="") {
    include($_SERVER['DOCUMENT_ROOT'] ."/login.inc.php");
} else { 
    $username=$_POST["username"];
    $password=$_POST["password"];
    session_start();
    if ($username=="bob" AND $password=="123"){ $permission="yes";}
    $username=$_POST["username"];
    session_register("permission");   
    session_register("username");  

    if ($permission=="yes"){
        // Show stuff
    }
}
?>
Copy code

Excuse the funky formatting of my code - can't seem to get it to show properly.

So, I keep getting this error:

Warning: Unknown: Your script possibly relies on a session side-effect which existed until PHP 4.2.3. Please be advised that the session extension does not consider global variables as a source of data, unless register_globals is enabled. You can disable this functionality and this warning by setting session.bug_compat_42 or session.bug_compat_warn to off, respectively in Unknown on line 0

Which I'm assuming means I need to change one of my variable names so it doesn't conflict with the session variable right? That's what I read, but I'm not sure which one to change.

Can anyone help / show me please?

Thanks

osu