Login User

  $user = get_user_by( 'id', $user_id ); 
  if ($user->ID > 0)
  {
    if (is_user_logged_in())
        wp_logout();

    wp_set_current_user($user_id, $user->user_login);     //This setting the current user does not log in that user. We also need the 2 following lines
    wp_set_auth_cookie($user_id);
    do_action('wp_login', $user->user_login);
  }

Logout User

  if (is_user_logged_in())
      wp_logout();