PHP无法获取类属性

class MyClass {
    public $publicProperty = 'Public Property';
}

$obj = new MyClass();
echo $obj->publicProperty; // 输出：Public Property

class MyClass {
    private $privateProperty = 'Private Property';

    public function getPrivateProperty() {
        return $this->privateProperty;
    }
}

$obj = new MyClass();
echo $obj->getPrivateProperty(); // 输出：Private Property

class MyClass {
    protected $protectedProperty = 'Protected Property';

    public function getProtectedProperty() {
        return $this->protectedProperty;
    }
}

class MyChildClass extends MyClass {
    public function getChildProtectedProperty() {
        return $this->protectedProperty;
    }
}

$obj = new MyClass();
echo $obj->getProtectedProperty(); // 输出：Protected Property

$childObj = new MyChildClass();
echo $childObj->getChildProtectedProperty(); // 输出：Protected Property