<?php
class Database extends Mysqli
{
    private static $_instance;

    const HOSTNAME = 'localhost';
    const USERNAME = 'root';
    const PASSWORD = '';
    const DATABASE = 'blog';

    private function __construct () { }

    private function __clone () { }

    protected static function getInstance ()
    {
        if (!isset(self::$_instance))
            self::$_instance = new parent(
                self::HOSTNAME,
                self::USERNAME,
                self::PASSWORD,
                self::DATABASE
            );
        return self::$_instance;
    }
}
?>