invoice.php

abstract class Invoice {
  abstract public function validate();
}

class EmailInvoice extends Invoice {
  public function validate() { }
}

class RealMailInvoice extends Invoice {
  public function validate() { }
}

class InvoiceFactory {
  static function createInvoice($type) {
    switch ($type) {
      case "RealMail":
        return new RealMailInvoice();
        break;

      case "Email":
        return new EmailInvoice();
        break;
      default:
        return null;
    }
  }
}

$invoice = InvoiceFactory.createInvoice($method);
$invoice.validate();