php mysql create

<?php
final class Constantes
{
    public static $db = null;
}

/**
 * @return mysqli
 */
function md_bd_connection()
{
    if (!Constantes::$db) {
        Constantes::$db = new mysqli('host', 'username', 'passw', 'dbname'); //TODO
    }
    return Constantes::$db;
}


#test
var_dump(md_bd_connection());
$db = md_bd_connection();
$res = $db->query("SELECT 3 AS TEST");
var_dump($res->fetch_assoc());