<?php
header ("content-type:text/html;charset=utf8");
class Car {
    private static $speed = 10;
    
    public static function getSpeed() {
        return self::$speed;
    }
    
    public static function speedUp() {
        return self::$speed+=10;
    }
}
class BigCar extends Car {
    public static function start() {
        parent::speedUp();
    }
}

BigCar::start();
echo BigCar::getSpeed();

静态方法1