<?php

class Car {

    public $speed = 0; //汽车的起始速度是0

    public function speedUp() {

        $this->speed += 10;

        return $this->speed;

    }

}

//定义继承于Car的Truck类

 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;

为什么  Truck 的speed 一直在增加