private 修饰的属性不能被继承和重写，protected可以

class Person
{
    private $name = 'KaiNan';
    protected $age = 22;

    public function getAge() {
        echo $this->age,'<br>';
        echo $this->name;
    }
}

class Stu extends Person
{
    private $name = 'NuoKe';
    protected $age = 18;

    public function intro()
    {
        parent::getAge();
    }
}

$lucy = new Stu();

$lucy->intro();

/*
18
KaiNan
*/