function process_demo_login(){
    if(isset($_GET['autologin']) && !is_user_logged_in()){
        $username=$_GET['autologin'];
        $password='demo';
        $user = wp_authenticate($username, $password);
        $secure_cookie = '';
        wp_set_auth_cookie($user->ID, true, $secure_cookie);
        do_action('wp_login', $username);
        wp_redirect(home_url());
    }
}

add_action('plugins_loaded','process_demo_login');