php5中抽象类的小例子

/**
* 定义与使用php抽象类
* edit: bbs.it-home.org
*/
     abstract class Number {
          private $value;
          abstract public function value();
          public function reset() {
               $this->value = NULL;
          }
     }
     class Integer extends Number {
          private $value;
          public function value() {
               return (int)$this->value;
          }
     }
     $num = new Integer; /* Okay */
     $num2 = new Number; /* This will fail */
?>