I've found some problems in your code:
PHP Code:
if (!isset($_SESSION['myusername']) || empty($_SESSION['myusername'])) {;
include "logout.php";
You have a semicolon ; after an opening brace - remove it.
PHP Code:
if(isset($_COOKIE['ID_mrtg'])) ;
Sei colon before the opening brace - remove it.
PHP Code:
<?php echo $myusername ?>
No semicolon at all - put one in.
You might also like to look at the link in my signature about coding styles and $end errors as it may be useful to you.