$_SESSION['authuser'] nedrží

session_unset(); 
    if ($_POST['loginout']=='Odhlásit') 
        {
            session_start();
            $_SESSION['authuser'] = 0;
        }
    if ($_POST['loginout']=='Přihlásit') 
        {
            session_start();
            $name = $_POST['login_name'];
            $pass = $_POST['login_pass'];
            $spojeni=MySQL_Connect('xxxxx','xxxxx','xxxxx');
                        MySQL_Select_DB('xxxxx');
                        mysql_set_charset("utf8");
            $dotaz = "SELECT * FROM  `users` WHERE  `username` = '$name'";
            $vysledek = mysql_query( $dotaz, $spojeni);
            
            while ($zaznam = MySQL_Fetch_Array($vysledek)) 
                {
                /*proměnná $pass se rovná tomu co bylo zadáno ve formulari s heslem*/
                if (($pass == $zaznam['heslo']) or ($_SESSION['authuser'] == 1)) 
                    {
                         $_SESSION['authuser'] = 1;
                    }
               else 
                    {
                           exit();
                    }
                }        
        }