怎么让wordpress随机文章不再随机变化

我用wordpress调用随机文章，下面的代码每次打开页面，随机文章都不一样，我想让文章产生的随机文章一次性随机生成后就不再变化了，请高手们帮我修改一下代码，好吗？万分感谢!代码如下：

 <?php $post_num = 4;
   $exclude_id = $post->ID;
    $posttags = get_the_tags();
    $i = 0;
    if ($posttags) {
        $tags = '';
        foreach ($posttags as $tag) $tags .= $tag->term_id . ',';
        $args = array('post_status' => 'publish',
            'tag__in' => explode(',', $tags),
            'post__not_in' => explode(',', $exclude_id),
            'ignore_sticky_posts' => 1,
            'orderby' => 'rand',
            'posts_per_page' => $post_num,
        ); query_posts($args);
        while (have_posts()) { the_post(); ?>
       <li>...</li>