PDO作为构造函数参数

我有以下课程:

class UserRepository {

    private $conn;

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

    //Methods omitted

}

我使用以下命令创建一个UserRepository对象:

$conn = new PDO("mysql:host=".DB_SERVER.";dbname=".DB_DATABASE, DB_USERNAME, DB_PASSWORD);
$userRepository = new UserRepository($conn);

Recoverable fatal error: Object of class PDO could not be converted to string

我做错什么了?