redirect_to how to make it simply work with get parameter or similar?

the issue was…

myappname_login_redirect needs to return $redirect_to in any case

otherwise the redirect to gets lost

background:

wp-login.php will render the redirect_to value into an input hidden into the login form.

so arriving at

wp-login.php?redirect_to=www.example.com

will render

an

<input type="hidden" value="www.example.com">

this value will be empty, if the filter does not return it by default

so fix…

   function myappname_login_redirect($redirect_to, $request, $user)
    {

      if ($user instanceof WP_User) {
        if (isset($user->roles) && is_array($user->roles)) {
          //check for admins
          if (in_array('administrator', $user->roles)) {
            // redirect them to the default place
            return $redirect_to;
          } else {
            //if something is set her for exceptions, we allow regular redirect as well
            if($_GET['myappname_allow_redirect']){
                return $redirect_to;
            }else{
//but normally we redirect to a standard url
                return 'somethingelse';
            }
          }
        } else {
          return $redirect_to;
        }
      }
      //is there a user to check?

return $redirect_to;
    }

    add_filter('login_redirect', 'myappname_login_redirect', 10, 3);