PHP help-dd3

I was looking at the Multiple drop down list box and I was wondering if you could help me or give suggestions... I want to use 2 drop down boxes for my website. But I want to use only one page instead of using an additional check page. It works fine... till I refresh the page. Then it asks for me to resend the form again. Is there a way to maybe clear the form? or on the refresh set the values to the previously submitted(resend the form)?

I did notice something. When I choose an option with my drop downs they do not appear in my url

ex) dd3.php?cat=2

my page never gives "?cat=2"