不得不说，当搜索结果只有一个时直接跳转到该文章这个功能很人性化哟：

add_action('template_redirect', 'redirect_single_post');
function redirect_single_post() {
    if (is_search()) {
        global $wp_query;
        if ($wp_query->post_count == 1 && $wp_query->max_num_pages == 1) {
            wp_redirect( get_permalink( $wp_query->posts['0']->ID ) );
            exit;
        }
    }
}

Have a try!