class db {
    static private $instance;
    private $host = 'localhost';
    private $user = 'username';
    private $password = 'password';
    private $db = 'database';
    private $mysqli;
    static public function getInstance() {
        if (!isset(self::$instance)) {
            self::$instance == new self();
        }
        return self::$instance;
    }
    public function __construct() {
        $mysqli = new mysqli($this->host,$this->user,$this->password,$this->db);
        if (mysqli_connect_errno()) {
            printf("Error %s\n".mysqli_connect_error());
            exit;
        }
    }
}