add_filter('posts_orderby_request', 'wpjam_search_orderby_filter');
function wpjam_search_orderby_filter($orderby = ''){
global $wpdb;
$keyword = $wpdb->prepare($_REQUEST['s']);
return "((CASE WHEN {$wpdb->posts}.post_title LIKE '%{$keyword}%' THEN 2 ELSE 0 END) + (CASE WHEN {$wpdb->posts}.post_content LIKE '%{$keyword}%' THEN 1 ELSE 0 END)) DESC, {$wpdb->posts}.post_modified DESC, {$wpdb->posts}.ID ASC";
}