why don't you use

$uri $is_args $args

This will build the URL like.

index.php ? arguement=value&moreargs=morevalue


$request_uri will always output the full URL. Not individual segments of it.

If you want the first part of the url only just use $uri on its own.

http://nginx.org/en/docs/http/ngx_http_core_module.html#var_uri

http://www.networkflare.com/