How to parse the parameters passed to a custom URL in a WP site

Hi

In my WP site-- eg. http://www.mydomain.com-- a request has to be passed to a custom URL-- http://www.mydomain.com/custompage?param1=abc&param2=def

Now, I wish to obtain the parameters passed to that URL from a function in functions.php of that theme. How do I do this? For above eg I wish to obtain first parameter=abc and second parameter=def-- I already know that the parameters are named param1 and param2.

Yours sincerely,
Arvind.