//Filter the search by title for posts
function filter_search_by_title($query){
    if($query->is_search() && $query->is_main_query()){
      $query->set('post_type', array('post'));
      $query->set('orderby', 'title');
      $query->set('order', 'ASC');
  }
  return $query;
}

add_action('pre_get_posts', 'filter_search_by_title');

function highlight_search_results($content){
     if(is_search()){
     $search_string = get_query_var('s');
     $keys = explode(" ",$search_string);
     $content = preg_replace('/('.implode('|', $keys) .')/iu', '<strong class="highligh-search">'.$search_string.'</strong>', $content);
     }
     return $content;
}
add_filter('the_excerpt', 'highlight_search_results');
add_filter('the_title', 'highlight_search_results');