如何运行mysqli查询？

       <?php

// Forum Configuration
define('DB_HOST','localhost'); // Database Hostname
define('DB_USER','root'); // Database Username
define('DB_PASSWORD',''); // Database Password
define('DB_NAME','ultraforum'); // Database Name
define('WEB_NAME','Website Name'); // Website Name
define('WEB_TITLE','Website Title'); // Website Title

// Do not modify anything under this line :)

    class db {

    var $dbhost;
    var $dbuser;
    var $dbpassword;
    var $dbname;
    var $query;

        function connect() {

            $this->db = 
                new mysqli($this->dbhost, $this->dbuser, $this->dbpassword, $this->dbname);

        }

        function __construct() {
            $this->dbhost = DB_HOST;
            $this->dbuser = DB_USER;
            $this->dbpassword = DB_PASSWORD;
            $this->dbname = DB_NAME;
        }

    }

    $con = new db;
    $con->connect();
    $con->query("SELECT * FROM topics");
       <?php

// Forum Configuration
define('DB