Adding to url with link

Adding to url with link

My url contains many variables that I want untouched (don't worry they
aren't important).
Let's say it contained...
../index.php?id=5
How would I make a url that just adds
&current=1
rather than replacing it entirely?
I'd like...
../index.php?id=5&current=1
rather than..
../index.php?current=1
I know it's a simple question but that's why I can't figure it out.
Thanks.