Mysql绑定插入类:无法插入oops

我只是学习类，我试图使用mysql绑定参数添加数据，但无法这样做，似乎有些愚蠢的错误，请帮助我，我想用oops的方式做到这一点，否则我可以做到。

错误:解析错误:语法错误，意想不到的'$mysqli' (T_VARIABLE)，期望函数(T_FUNCTION)在customer.php第4行

index . php

$adduser= new Userclass();
echo $adduser->addcutomer();

Db class.php

class db extends mysqli {
    private static $instance = null;
    private $user = "root";
    private $pass = "xxx";
    private $dbName = "xxx";
    private $dbHost = "localhost";
    public static function getInstance() {
    if (!self::$instance instanceof self) {
            self::$instance = new self;
    }
        return self::$instance;
    }
    public function __clone() {
        trigger_error('Clone is not allowed.', E_USER_ERROR);
    }
    public function __wakeup() {
        trigger_error('Deserializing is not allowed.', E_USER_ERROR);
    }
    private function __construct() {
        parent::__construct($this->dbHost, $this->user, $this->pass, $this->dbName);
        if (mysqli_connect_error()) {
            exit('Connect Error (' . mysqli_connect_errno() . ') '
                    . mysqli_connect_error());
        }
        parent::set_charset('utf-8');
   }
    public function dbquery($query)
    {
        if($this->query($query))
        {
            return true;
        }
    }
}

客户类

class Userclass extends db{
  $mysqli=db::getInstance();
  public function addcutomer(){
    $query = $mysqli->prepare("INSERT INTO `user` (`firstname`,`lastname`) VALUES (?, ?)");
    $col1 = 'abc';
    $col2 = 'def';
    $query->bind_param('ss', $col1, $col2);
    $query->execute();
    $query->close();
    return true;
  }