Working with question mark in url in codeigniter routing

Once i was working with Paypal payment integration (subscriptions) with codeigniter. Got the subscription part working but i was having problem when paypal redirected to my site after a successful payment.

My return url after successful payment from paypal was controller_name/get_success,
but Paypal added some extra info in this url and made it like:
controller_name/get_success?auth=some_auth_code
Since i had used

     $config["enable_query_string"] = FALSE;
?>

this caused me problem, it was showing a 404 page. I tried fixing this problem with routing like,

    $route["controller_name/get_success\?(:any})] = "controller_name/get_success";
?>

but this didnt work at all.
So, after some searching, i came up with a solution, and it was using hooks. Though we could use pre_controller hooks, but i opted using a pre_system hook, as:

Step 1:
//Inside application/config/config.php

    $config['enable_hooks'] = TRUE;
?>

Step 2:
//inside application/config/hooks.php

$hook['pre_system'] = array(
   'function' => 'remove_the_stuff',
   'filename' => 'remove_get.php',
   'filepath' => 'hooks'
        );
?>

Step 3:
Created remove_get.php file inside application/hooks

    function remove_the_stuff() {
       if (isset($_GET['auth'])) {
       define('AUTH', $_GET['auth']);
    unset($_GET);
       } 
    }
?>

Step 4:
Set up the uri_protocol in application/config/config.php

    $config["uri_protocol"] = "REQUEST_URI";
?>

And thats it, its working without any problems.Hope this is useful.