Query string to variable?

I’m a front end developer, and need to do something which is really beyond my know-how, so please be patient with me if I’m talking arse.

The site I’m building is going to have an affiliate program, powered by AffiliateShop. We want to set up direct linking, which AffiliateShop enable with something called Flexilink.

I theorise that it ought to be possible to generate the img tag they want (something along the lines of <img src="https://edge.affiliateshop.com/public/FlexiLink?AID=123456" />) by taking the query string straight from the URL (eg http://www.mydomain.com/slug/?AID=123456) and turning it into a variable to spit out the affiliate ID (AID) in the appropriate place, with some relatively straightforward php rather than messing about with perl.

But I have no idea how to do it. (And also I wonder whether the permalinks would get in the way.)

If anyone can tell me how to do this (or if it’s possible) I’d really appreciate it – again, I’m a front-end developer, so bear in mind I have only a basic grasp of php and be gentle with me!