<?php if ($gebruiker->checkLogout()) { ?>
  <li><a href="logout.php">Loguit</a></li>
<?php } ?>

function checkLogout(){
       if (isset($_SESSION['login'])) {
         return true;
       }else {
         return false;
       }
     }

---------------
Logic for login
---------------

------------
Menu Section
------------

<?php if($_SESSION['login'])

echo <a href="logout.php">Loguit</a></li>';
else{
echo '<li><a href="login.php">Login/Registreren</a></li>';
}
?>