One URL parameter without a name

My site currently uses one parameter in the URL like this:

http://www.mydomain.com/folder/?paramname=AB32123

I would like to remove the parameter name, so the URL is:

http://www.mydomain.com/folder/?AB32123

How would I then call use this parameter,

currently I use $_GET['paramname'] but without a name I am unsure how

I have tried:

$_GET

$_GET[]

but they both through up errors?

what is the correct way to get this paramter without a name