建立一个Truck类，扩展Car类，并覆盖speedUp方法，使速度累加50
<?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;