class Car {
   private static $speed = 10;
   
   public static function getSpeed() {
       return self::$speed;
   }
   
   public static function speedUp() {
       return self::$speed+=10;
   }
}
class BigCar extends Car {
   public static function start() {
       parent::speedUp();
   }
}BigCar::start();echo BigCar::getSpeed();

我可不可以吧BIGCAR也写成静态的方法？然后去调用speedUp()?如果能要怎么写？