Register and login with php & mysqli using oop

connection.php

<?php
// filename include/connection.php
define('DB_SERVER', 'localhost');
define('DB_USERNAME', 'root');
define('DB_PASSWORD', '');
define('DB_DATABASE', 'oop');

function db () {
    static $conn;
    if ($conn===NULL){ 
        $conn = new mysqli(DB_SERVER, DB_USERNAME, DB_PASSWORD, DB_DATABASE);
    }
    return $conn;
}