filter search results

Hi there,

I’m trying to work out how to filter search results. I am using this search form on a category listing page:

<form method="get" id="searchForm" action="<?php get_bloginfo('wpurl') ?>" ><fieldset><input type="search" placeholder="Search within results" name="s" value="" /><input type="hidden" value="3" name="cat" id="scat" /></fieldset></form>

Results are displayed using search.php – in search.php i want to take the existing search, and filter within it. i think it should be possible to grab the existing search from the url and just add the new search into the url.. but i’m not sure how!

any ideas? thanks!