在尝试向db插入数据时，获取“可捕获的致命错误：类PDOStatement的对象无法转换为字符串”

I just don't know why i am getting this err. Expert please help me out. I googled and seen very similar type of problem in SO also but I couldn't figure out what actually wrong in my code -- I became helpless here.

My code snippest is given below :

Database:

class Database extends PDO
{
    function __construct($dbtype,$dbhost,$dbname,$dbuser,$dbpass)
    {
        parent::__construct($dbtype.':host='.$dbhost.";dbname=".$dbname,$dbuser,$dbpass);
    }
}

Model :

<?php

/**
* Base Model Class
*/
class Model
{
    function __construct()
    {
        $this->db = new Database(DB_TYPE,DB_HOST,DB_NAME,DB_USER,DB_PASS);
    }
}

Controller:

<?php

/**
* Auth Controller
*/
class RegistrationController extends Controller
{

    function __construct()
    {
        parent::__construct();
    }

    public function index()
    {
        $this->view->render('registration');
    }

    public function registration()
    {
        $data = array();

        $data['username'] = $_POST['username'];
        $data['email'] = $_POST['email'];
        $data['contact'] = $_POST['contact'];
        $data['password'] = Hash::create('md5', $_POST['password'], HASH_PASSWORD_KEY);

        $this->model->registration($data);
    }

}

Registration Model :

class Registration extends Model
{

    function __construct()
    {
        parent::__construct();
    }

    public function registration($data)
    {
        $sql = $this->db->prepare("INSERT INTO users (username, email, contact ,password) VALUES (:username, :email, :contact, :password)");
        $sql->execute(array(
            ':username' => $data['username'],
            ':email' => $data['email'],
            ':contact' => $data['contact'],
            ':password' => $data['password']
            ));
    }
}