I’ve found a couple ways to do this. You can either tell your server to rewrite a *.css request to look at a PHP script (such as “style.php?name=style”). The other way you can do this is by configuring .css files to be handled by php. I think this .htaccess code should work:
php_value default_mimetype "text/css"
Make sure this .htaccess is at least in the same directory as your css php script.