<?php

class Car {

? ? public $speed = 0; //汽車的起始速度是0

? ??

? ? public function speedUp() {

? ? ? ? $this->speed += 10;

? ? ? ? return $this->speed;

? ? }

}

//定義繼承于Car的Truck類

?class Truck extends Car {

? ? public function speedUp() {

? ? ? ? $this->speed = parent::speedUp() + 50;

? ? }

}?

$car = new Truck();

$car->speedUp();

echo $car->speed;

?$car->speedUp();

?echo $car->speed;

? $car->speedUp();

?echo $car->speed;

為什么 ?Truck 的speed 一直在增加?