How to return the names of $_GET variables in a url php

I want to save the name of all the $_GET variables in a url, but im not sure where to start, or finish for that matter.

For example:

if i have:

url: test.com/forums.php?topic=blog&discussion_id=12

can i use php to get the name, i.e. "topic" and "discussion_id from the $_GET variables and can i then store the values: "topic" and "discussion_id" in an array?