WordPress:Warning: Missing argument 2 for wpdb:: prepare()

Question

add_filter('posts_orderby_request', 'wpjam_search_orderby_filter');
function wpjam_search_orderby_filter($orderby = ''){
if(is_search()){
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";
}else{
return $orderby;
}
} 

提示错误：Warning: Missing argument 2 for wpdb::prepare()，就是

$keyword = $wpdb->prepare($_REQUEST['s']);  

WordPress官方提示 http://make.wordpress.org/core/2012/12/12/php-warning-missing-argument-2-for-wpdb-prepare/

不懂代码，压力山大啊，求修正。

Answer 1

将

$keyword = $wpdb->prepare($_REQUEST['s']); 

改为

$keyword = $wpdb->prepare($_REQUEST['s'],'');