致命错误:即使在对象中，也不在对象上下文中使用$this

我在调用数据库类并运行查询函数时收到此错误。

这是我的班级：

class dbConnect {
public $mysqli = null;
public function __construct() {
    $this->mysqli = new mysqli( "localhost", "root", "pass", "database" );
    if ($this->mysqli->connect_errno) 
    {
        echo "Error MySQLi: ("&nbsp. $this->mysqli->connect_errno 
        . ") " . $this->mysqli->connect_error;
        exit();
    }
    $this->mysqli->set_charset("utf8"); 
}
public function __destruct() {
    $this->closeDB();
}
public function runQuery($qry) {
    $result = $this->mysqli->query($qry);
    return $result;
}

这是我的页面：

$db = new dbConnect();
$insert = "INSERT INTO `users` (`username`,`password`,`email`,`ip`,`date`) VALUES ('$username','$pw','$email','$ip','$time')";
$db->runQuery( $insert ) or die("Error MySQLi: ("&nbsp. $db->mysqli->connect_errno . ") " . $db->mysqli->connect_error);

编辑：

我现在收到错误 Dbr'0