<?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;