<?php

class Car {

    private static $speed = 10;

    public static  function getSpeed() {

        return self::$speed;

    }

}

class See extends Car{

public static  function getSpeed() {

return parent::$speed;

    }

}

echo See::$speed ; 

?>

上述报错，但是把输出改为See::getSpeed()就可以，为什么