Custom class for Database connecting and executing sql query

CosDb.php

<?php
class CosDb {
    private $DB_HOST;
    private $DB_PORT;
    private $DB_NAME;
    private $DB_USER;
    private $DB_PASS;
    private $link;

    public function __construct($user, $pass, $db_name, $host = 'localhost', $port = null) {
        $this->setInfo($user, $pass, $db_name, $host, $port);
        $this->link = mysqli_connect($this->DB_HOST, $this->DB_USER, $this->DB_PASS, $this->DB_NAME, $this->DB_PORT);

        if (!$this->link || $this->link->connect_errno) {
            printf("Connect failed: %s\n", $mysqli->connect_error);
            exit();
        }
    }

    private function setInfo($user, $pass, $db_name, $host, $port)
    {
        $this->DB_HOST = $host;
        $this->DB_PORT = is_null( $port ) ? ini_get("mysqli.default_port") : $port;
        $this->DB_NAME = $db_name;
        $this->DB_USER = $user;
        $this->DB_PASS = $pass;
    }

    public function query($sql)
    {
        $this->link->query("SET NAMES utf8;");
        $result = $this->link->query($sql);
        return $result;
    }

    public function close()
    {
        $this->link->close();
    }

    public function real_escape_string($value='')
    {
        return $this->link->real_escape_string($value);
    }
}