PHP“致命错误：未捕获错误：调用成员函数prepare（）为null”

Question

我正在观看YouTube视频，以使用php制作上市网站。但是，当我尝试创建新列表时，这会在我的浏览器中发生：

致命错误：未捕获的错误：调用成员函数prepare（）空入D： Dev Dependencies XAMPP htdocs addonlister lib Database.php：31堆栈跟踪：＃0D： Dev Dependencies XAMPP htdocs addonlister lib Addon.php（61）：数据库->查询（'SELECT * FROM a ...'）＃1D： Dev Dependencies XAMPP htdocs addonlister addon.php（10）：抛出Addon-> getAddon（'2'）＃2 {main}D： Dev Dependencies XAMPP htdocs addonlister lib Database.php在线31

这是我在参考页上的代码：

database.php

<?php
class Database{
private $host = DB_HOST;
private $user = DB_USER;
private $pass = DB_PASS;
private $dbname = DB_NAME;

private $dbh;
private $error;
private $stmt;

public function __construct(){
    // Set DSN
    $dsn = 'mysql:host=localhost'. $this->host .';dbname=addonlister' . $this->dbname;

    // Set Options
    $options = array(
            PDO::ATTR_PERSISTENT => true,
            PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION
    );

    // PDO Instance
    try{
        $this->dbh = new PDO($dsn, $this->user, $this->pass, $options);
    } catch(PDOException $e) {
            $this->error = $e->getMessage();
    }
}

public function query($query){
        $this->stmt = $this->dbh->prepare($query);
}

public function bind($param, $value, $type = null){
        if(is_null($type)){
                switch(true){
                    case is_int ( $value ) :
                        $type = PDO::PARAM_INT;
                        break;
                    case is_bool ( $value ) :
                        $type = PDO::PARAM_BOOL;
                        break;
                    case is_null ( $value ) :
                        $type = PDO::PARAM_NULL;
                        break;
                    default :
                        $type = PDO::PARAM_STR;
                }
        }
        $this->stmt->bindValue($param, $value, $type);
}

public function execute(){
    return $this->stmt->execute();
}

public function resultSet(){
    $this->execute();
    return $this->stmt->fetchAll(PDO::FETCH_OBJ);
}

public function single(){
    $this->execute();
    return $this->stmt->fetch(PDO::FETCH_OBJ);
}}

Addon.php

<?php
class Addon{
    private $db;

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

    // Get All Addons
    public function getAllAddons(){
        $this->db->query("SELECT addons.*, categories.name AS cname
                FROM addons
                INNER JOIN categories
                ON addons.category_id = categories.id
                ORDER BY post_date DESC
                ");
        // Assign Result Set
        $results = $this->db->resultSet();

        return $results;
    }

    //Get Categories
    public function getCategories(){
        $this->db->query("SELECT * FROM categories");
        // Assign Result Set
        $results = $this->db->resultSet();

        return $results;
    }

    // Get Addons By Category
    public function getByCategory($category){
        $this->db->query("SELECT addons.*, categories.name AS cname
                     FROM addons
                     INNER JOIN categories
                     ON addons.category_id = categories.id
                     WHERE addons.category_id = $category
                     ORDER BY post_date DESC
                     ");
       // Assign Result Set
       $results = $this->db->resultSet();

       return $results;
    }

    // Get category
    public function getCategory($category_id){
            $this->db->query("SELECT * FROM categories WHERE id = :category_id"
                );
            $this->db->bind(':category_id' , $category_id);

            // Assign Row
            $row = $this->db->single();

            return $row;
    }

    // Get Addon
    public function getAddon($id){
        $this->db->query("SELECT * FROM addons WHERE id = :id");

        $this->db->bind(':id' , $id);

        // Assign Row
        $row = $this->db->single();

        return $row;
    }

    // Addon Job
    public function create($data){
        //Insert Query
        $this->db->query("INSERT INTO addons (category_id, addon_title, description, developer, file_size)
        VALUES (:category_id, :addon_title, :description, :developer, :file_size)");
         //Bind Data
        $this->db->bind(':category_id', $data['category_id']);
        $this->db->bind(':addon_title', $data['addon_title']);
        $this->db->bind(':description', $data['description']);
        $this->db->bind(':developer', $data['developer']);
        $this->db->bind(':file_size', $data['file_size']);
        //Execute
        if($this->db->execute()){
            return true;
        } else {
            return false;
        }
    }
}

Addon.php

<?php include_once 'config/init.php'; ?>

<?php
$addon = new Addon;

$template = new Template('templates/addon-single.php');

$addon_id = isset($_GET['id']) ? $_GET['id'] : null;

$template->addon = $addon->getAddon($addon_id);

echo $template;

谢谢！