Hola,
No tendria que ser haci la parte de logout:
Código PHP:
<?php
session_start();
include("includes/connect.php");
$lastlogin=date("d/m/Y h:i");
$username=($_SESSION['s_username']);
$query = "DELETE FROM online WHERE username ='$username' ";
mysql_query($query);
session_destroy();
exit;
?>
suerte
salu2