How to prevent usage of public parent methods in child instance in PHP?

Question

So let's say I have classes called parent and child, which will be then used from PHP file called caller.php

class Child extends Parent {

}

class Parent {

    public function parentMethod(){

    }
}

caller.php

PREVENTED:

$child = new Child();
$child->parentMethod();

ALLOWED:

$parent = new Parent();
$parent->parentMethod();

I want to prevent calling parentMethod like this. But if I created Parent object I want to be able to call the parentMethod. Is there some way that I can use to hide this method from being public in Child class, but still allowing parent object to call this method publicly?

Only solution I have come up with so far is making those methods protected and then creating an other class that would extend parent and then have public method for each function that it needs, but that doesn't sound very smart.

Answer 1

Actually, you should ask yourself: why do you need such restriction? You've defined your method as public - thus, you told PHP that it should be visible everywhere. So to prevent child calls you should use private visibility definition.

There is a way to check if call is made from parent class, like:

class ChildClass extends ParentClass {}

class ParentClass 
{

    public function parentMethod()
    {
       if(get_class($this) != __CLASS__)
       {
          throw new LogicException("Somehow due to business logic you're not allowed to call this from childs");
       }
    }
}

But I would not recommend to do that. Reasons are:

• Readability. Your method is just ordinary public method. Looking to it it's impossible to say either you should use it with child calls or not. Thus, to maintain such code you'll need to check that restriction in code. Now imagine that you have ~50 methods like that. And dozen of classes like that.
• Possibly, breaking Law of Demeter. Why should parent class be aware of it's childs when using such limitation?
• Finally, it's just unexpected behavior. Looking to definition, anybody will see that you're extending one class by another. Thus, by definition all inherit methods with proper visibility must be inherited. And your logic changes that.

You may think about composition, not inheritance. That may be right way to implement your logic (however, I can't tell that for sure since I don't know whole background)