OOP PHP数组到字符串转换错误

I need a little help here, please. It shows "Array to String conversion" error in my frontpage.php But if I use a print_r($topics) it displays all the results from the database which is my class Topic doing I think my __toString() method is not working. Could you help me to understand please. Thank you. This is my code.

My frontpage file

<?php 
    foreach($topics as $topic) {
        echo $topic->title;
    }

My index file

<?php require('core/init.php'); ?>

<?php 

//Create Topic Object
$topic = new Topic;


//Get Templates & Assign Vars
$template = new Template('templates/frontpage.php');

//Assgin Vars
$template->topics = $topic->getAllTopics();

//Display template
echo $template;

My init file

require_once('config/config.php'); // it has the database host, username, password


function __autoload($class_name) {
require_once('libraries/'.$class_name.'.php'); //it has my classes: Template, Topic
}

My class Template

class Template {
    protected $template;
    protected $vars = array();

    public function __construct($template) {
        $this->template = $template;
    }

    public function __get($key) {
        return $this->vars[$key];
    }

    public function __set($key, $value) {
        $this->vars[$key] = $value;
    }

    public function __toString() {
        extract($this->vars);
        chdir(dirname($this->template));
        ob_start();

        include basename($this->template);

        return ob_get_clean();
    }

My Class Topic

class Topic{
//Initialize DB Variable
private $db;

//Constructor
public function __construct() {
    $this->db = new Database();
}

//Get All Topics
public function getAllTopics() {
    $this->db->query("SELECT topics.*, users.username, users.avatar, categories.name FROM topics
                        INNER JOIN users
                        ON topics.user_id = users.id
                        INNER JOIN categories
                        ON topics.category_id = categories.id
                        ORDER BY create_date DESC
                        ");

    //Assing Results Set
    $results = $this->db->resultset();

    return $results;
}