How to configure Apache for XAMPP on Mac OS X Lion to use mod_rewrite?
I haven't tested this exact string but in your htaccess file you can try:

Options +FollowSymLinks

RewriteEngine On

 RewriteCond %{QUERY_STRING} ^(.*&amp;)category=([^&amp;]+)(&amp;.*)?$ [NC]

 RewriteRule ^category\.php$ /%2? [R=301,L]


How familiar are you with regular expressions?  That's probably the biggest hurdle to get over with mod_rewrite.  

This particular version is intended to work for multiple parameters (optionally) and category can be first or not first.   The other parameters if present are just discarded -- they still sent to your php code, just don't appear in URL.