I started once to write something like that but i never finished it and always customize it for each client that needs this functionality
so enjoy and feel free to use/change any way you want
<?php
/*
Plugin Name: BaInternet Search Filters
Plugin URI: http://www.bainternet.info
Description: filter searches by tags and categories
Version: 0.0.1
Author: bainternet בניית אתרים
Author URI: http://www.bainternet.info
*/
/*
* very early beta so watch out.
*
* Usage: add [search_with_filter] as shortcode to
* post/page or widget or call it from a template
* page like so: echo do_shortcode('search_with_filter');
*
*
*/
function ba_SearchFilter($query) {
if (!$query->is_search) {
return $query;
}
if (isset($_POST['cat'])){
$query->set('category__and', $_POST['cat']);
}
if (isset($_POST['tags'])){
$query->set('tag__and', $_POST['tags']);
}
return $query;
}
//hook filters to search
add_filter('pre_get_posts','ba_SearchFilter');
function ba_search_with_filters(){
$out = '<form role="search" method="get" id="searchform" action="'. home_url( '/' ).'">
<div><label class="screen-reader-text" for="s">Search for:</label>
<input type="text" value="" name="s" id="s" /><br />';
$categories= get_categories();
foreach ($categories as $category) {
$option = '';
$option .= '<input type="checkbox" name="cat[]" id="cat[]" value="'.$category->term_id.'"> ';
$option .= $category->cat_name .'<br />';
$out.= $option;
}
$tags= get_categories();
foreach ($tags as $tag) {
$option = '';
$option .= '<input type="checkbox" name="tags[]" id="tags[]" value="'.$tag->term_id.'"> ';
$option .= $tag->cat_name .'<br />';
$out.= $option;
}
$out .='<input type="submit" id="searchsubmit" value="Search" />
</div>
</form>';
return $out;
}
add_shortcode('search_with_filter','ba_search_with_filters');
?>