__construct() 개인적인 메소드

private __construct ( string $amount, Currency $currency )
$amount string Amount, expressed as a string (eg '10.00')
$currency Currency
 /**
  * @param integer $amount
  */
 public function __construct($amount, Currency $currency = null)
 {
     if ($currency === null) {
         $currency = new Currency(self::DEFAULT_CURRENCY_NAME);
     }
     parent::__construct($amount, $currency);
 }