存储的数据库连接变量在文件中，但标记为对象

Question

我有这个数据库类连接到MySQL：

private $db_host;
private $db_user;
private $db_pass;
private $db_name;

function __construct($db_host, $db_user, $db_pass, $db_name) {
    $this->db_host = $db_host;
    $this->db_user = $db_user;
    $this->db_pass = $db_pass;
    $this->db_name = $db_name;
}

public function connect(){
    if(!$this->con){
        $this->myconn = new mysqli($this->db_host,$this->db_user,$this->db_pass,$this->db_name);
        if($this->myconn->connect_errno > 0){
            array_push($this->result,$this->myconn->connect_error);
            return false;
        }else{
            $this->con = true;
            return true;
        } 
    }else{  
        return true;
    }   
}

我将$db_server, $db_user, $db_password, $db_name存储到config.php文件中。

每当我想连接到数据库时，我都会使用以下代码：

require_once ($_SERVER['DOCUMENT_ROOT'] . '/config.php');
$product = new Product($db_server, $db_user, $db_password, $db_name);
$product->connect();

不幸的是，那段代码产生了这个错误日志：

[06-Jan-2017 12:37:50 Asia/Jakarta] PHP Warning:  mysqli::mysqli() expects parameter 2 to be string, object given in /home/**/public_html/model/Database.class.php on line 39

第39行包含：

$this->myconn = new mysqli($this->db_host,$this->db_user,$this->db_pass,$this->db_name);

为什么$db_server, $db_user, $db_password, $db_name变量标记为对象？如何将其更改为字符串？这样错误就不会再出现了。

谢谢。