<class Car {
public $speed = 0; //汽车的起始速度是0
public function speedUp() {
$this->speed += 10;
return $this->speed;
}
}
class Truck extends Car{
public function speedUp(){
return $this->speed+=50;
}
}
$car = new Truck();
$car->speedUp();
echo $car->speed;
