.htaccess for short URL redirection on a subdomain?

I have an application that when a new user registers, it creates a folder for that user: /users/$user_id

It then writes an index.php file to that directory, that contains the following code:

<?php
define('GIVE', 'TRUE');
define('GIVE_PATH', dirname(__FILE__));

require(dirname(dirname(GIVE_PATH)) . '/gp_wrapper.php');
?>

It uses the path info to get the users ID for loading the necessary info.

Now, I have a subdomain that redirects to the users folder...

So visiting subdomain.mydomain.com/user_id/ will perform the functions I want for that user...

What I want to do however, is eliminate the need for a user/id folder structure entirely.

I know there is a way that I can use mod_rewrite to have it so that when someone visits subdomain.mydomain.com/user_id/ it will invisibly redirect to www.mydomain.com/index.php?mode=give&id=$id... however I am not very experienced with mod_rewrite and am running into problems...

This is what I have come up with so far... but it is giving me a 404 error.

Options +FollowSymlinks
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} !www.mydomain.com
RewriteCond %{HTTP_HOST} subdomain.mydomain.com
RewriteRule .* www.mydomain.com/index.php?mode=give&id=%1

It may also be worth noting, that because this domain that the application is hosted on, it is not actually located in the root public_html folder on my server, where .htaccess is located.

Rather, the root of the domain is at public_html/www.mydomain.com/

Thanks in advance.

I actually was able to solve the problem by creating a new .htaccess file in the root of my application.

<IfModule mod_rewrite.c>
    Options +FollowSymlinks
    RewriteEngine On
    RewriteCond %{HTTP_HOST} give.mydomain.com
    RewriteRule ^([0-9]+)$ /index.php?mode=give&id=$1
</IfModule>