Set your URI protocol to REQUEST_URI in application/config/config.php , like this:
$config['uri_protocol'] = 'REQUEST_URI';
then use GET method:
$this->input->get('a');
EDIT:
Since http://example.com/test?a=1&b=2 is not encoded URL, it isn't possible. So first, I would encode URL with urlencode function like this:
urlencode('http://example.com/test?a=1&b=2');
it returns something like: http%3A%2F%2Fexample.com%2Ftest%3Fa%3D1%26b%3D2
So I would pass the URL like this:
http://myurl.com/?url=http%3A%2F%2Fexample.com%2Ftest%3Fa%3D1%26b%3D2
then get an example URL with GET method.
$this->input->get('url');