面向对象的PHP，从方法返回时数组为null

尝试开发面向对象的PHP。我有一个递归方法来返回注释及其回复，并将它们编译为平面数组$comments_list。

<?php
class RedditPosts
{
    public function get_post_ids($from, $limit)
    {
        // GET POSTS
        $list = json_decode(file_get_contents("http://www.reddit.com/$from.json?limit=$limit"));
        sleep(2); //after every page request
        $post_ids = array();
        foreach($list->data->children as $post) {
            $post_ids[] = $post->data->id;
        }
        return $post_ids;
    }
}
class RedditComments
{
    static $comments_list = array();
    public function get_comments($post_id)
    {
        $comments_object = json_decode(file_get_contents("http://www.reddit.com/comments/$post_id.json"));
        sleep(2);
        $top_comments = $comments_object[1]->data->children;
        //var_dump($top_comments);
        self::get_sub_comments($top_comments);
    }
    static function get_sub_comments($root_comments)
    {
        foreach($root_comments as $comment)
        {
            self::$comments_list[] = $comment;
            //echo $comment->data->body . "<br/>"
            if ($comment->data->replies != '')
            {
                self::get_sub_comments($comment->data->replies->data->children);
            }
        }
        var_dump(self::$comments_list);
        return self::$comments_list;
    }
}
/******************************MAIN************************************/
$ps = new RedditPosts();
$my_posts = $ps->get_post_ids("r/learnprogramming", 2);
$cm = new RedditComments();
$my_comments = $cm->get_comments($my_posts[0]);
var_dump($my_comments);
?>

我在返回之前做了一个var_dump，它被填充了，看起来是正确的，但当我在方法外部调用它时，它是null。可能是范围的问题，但我是新手，不知道在哪里，我遇到了麻烦。感谢帮助！