SoundCloud API: redirect url - soundcloud

Question

Consider this example for authentication from PHP at https://developers.soundcloud.com/docs/api/guide#authentication seem to suggest you can pass a redirect url as a parameter when you flow a user throught the authentication process:
require_once 'Services/Soundcloud.php';
// create client object with app credentials
$client = new Services_Soundcloud(
'CLIENT_ID', 'CLIENT_SECRET', 'REDIRECT_URL');
// redirect user to authorize URL
header("Location: " . $client->getAuthorizeUrl());
Note the 'REDIRECT_URL' argument in the call to the constructor.
That seems to suggest I can pass an arbitrary redirect url as a parameter, just like you can do with Twitter (the API is quite similar).
However, if I pass an url that does not match the unique redirect url configured for the application, I get an error when the user is redirected to my url:
error=redirect_uri_mismatch&error_description=The+redirection+URI+provided+does+not+match+a+pre-registered+value.
So, what is that parameter supposed to be for, if the only valid value is the redirect url configured for the application?
And how are you supposed to handle authentication if the user can only be redirected to a single fixed url after authentication?? That makes the API completely unusable. When you have a user login into any API (e.g. Twitter or Facebook), you need them to be returned to the page from which they clicked the link to log in, and it is a ridiculous restriction that that url be unique. No other social network api that I've ever seen has this restriction.
Is SoundCloud API really so flawed or am I missing something?

Answer 1

I got an answer from Glen Scott, the author of the php-soundcloud library (a pretty decent wrapper around this terrible API) who provides a workaround. It's painful as it involves an additional redirect but it's all the API allows.
https://github.com/mptre/php-soundcloud/issues/36
I quote:
The API does not allow an arbitrary URL. As you noted, this is unlike
most other oAuth-backed social network API's. The workaround I can
recommend is using the state parameter to pass back your return URL.
You can do this when generating the authorization URL like this:
$client->getAuthorizeUrl(
array(
'state' => 'http://example.com/return'
) You'll get the state parameter added to the static redirect URL. For example, if you set your redirect URL to
http://example.com/callback then SoundCloud will redirect an
authenticated user to
http://example.com/callback?state=http://example.com/return
Your callback script will need to look for the state GET parameter,
and redirect using it.