Multiple Category Search Storm

Instalación

1. Upload this plugin to your WordPress plugin directory( generally wp-content/plugins/ ).
2. Activate it in admin area.
3. Your theme should have a hardcoded search form or you should add a «Search» widget to sidebar to allow visitors searching.
   Search Storm is working now!
4. If Search Storm doesn’t work, check whether or not there is a «searchform.php» file in your current theme directory, if not, you don’t need do anything; If yes, edit this «searchform.php» following the below instructions:

Your theme’s «searchform.php» file may looks like below, with some other codes:

....
<form method="get" id="searchform" action="<?php bloginfo('url'); ?>">
    <input type="text" id="s" name="s" value="<?php the_search_query(); ?>"  />
    <input type="submit" id="searchsubmit" value="Search" />
</form>
...

add the <?php if(function_exists('wss_add_form_field')) wss_add_form_field(); ?> code to proper place inside <form> element, in most cases you can put it just after <input type="submit" value="Search" /> button and before </form> close tag.

That’s all. You may want to add css styles to make search form nicer.