Ошибка с сессиями

<?php
include './include/config.php'; 
    
$connect = mysql_connect($host, $user, $pass);
        mysql_select_db($db);
            mysql_query('SET NAMES cp1251');
?>
 
 
<?php
 
if (isset($_POST['login']))
{
$login = mysql_real_escape_string($_POST['login']);
$password = mysql_real_escape_string($_POST['password']);
 
$result = "SELECT * FROM `users` WHERE `login` = '$login' AND `password` = '$password'";
$res = mysql_query($result) or die(mysql_error());
 
if ($row = mysql_fetch_assoc($res))
{
session_start();
$_SESSION['user_id'] = $row['id'];
$_SESSION['ip'] = $_SERVER['REMOTE_ADDR'];
}
header("Location: http://".$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI']);
exit;
}
if (isset($_GET['action']) AND $_GET['action']=="logout") {
  session_start();
  session_destroy();
  header("Location: http://".$_SERVER['HTTP_HOST']."/");
  exit;
}
if (isset($_REQUEST[session_name()])) session_start();
if (isset($_SESSION['user_id']) AND $_SESSION['ip'] == $_SERVER['REMOTE_ADDR']) return;
else
{
 
?>
 
<form method="POST">
<input type="text" name="login"><br>
<input type="password" name="password"><br>
<input type="submit" value = " Vhod "><br>
</form>
 
<?php
 
}
 
exit;
 
?>