将查询字符串变量传递给WordPress页面模板

Question

I have a fully-functional WordPress site with a URL structure like this:

http://example.com/friendlyurl/

But I would like to be able to pass an ID number like this (or whatever URL structure would work):

http://example.com/friendlyurl/?id=123

I have a custom page template that will handle everything from there, I just need a simple way to get the number to the page template. I.e.:

$id=trim($_GET["id"]);

I am not a WordPress whiz, just looking for a simple solution here to solve a problem. Would appreciate any advice or direction.

Thank you!