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<!-- If you want to have a deeper control over the results users can get, you might need a way not to show specific categories within the results page. The use cases are endless: promotional material, press release, translated content, portfolio items, just to name a few. Interested in how to do that? Open your functions.php file and add: -->

function SearchFilter($query) {
  if ( $query->is_search && ! is_admin() ) {
    $query->set('cat','8,15'); 
  }
  return $query; 
}
add_filter('pre_get_posts','SearchFilter');

<!-- With the possibility to exclude specific categories from your search results page, you'll also improve the quality of the information you deliver to your users and readers. -->