How to make available only those public methods which are declared in interface?

Question

In Java, for example, this comes out of the box due to static nature of the language. But in PHP this could be also useful especially to keep a code secure and clean from an architectural point of view.

For example, we have the following code:

interface DocumentProcessorInterface

   public function process();

}

class GameSaveProcessorImpl implements DocumentProcessorInterface {

    public function process() {
        // do something useful
    }

    public function methodToSetSomethingFromFriendClass() {
       // setting private fields, some postponed initialization/resetting, etc
    }

}

Then, from some class (lets call it "friend class", because it lies side-by-side with the GameSaveProcessorImpl class and compelements it) we call method methodToSetSomethingFromFriendClass. Because of PHP duck typing such ability to call this method is also available to any alien client code, but this method is not for such usage. Alien (external) client code MUST use only DocumentProcessorInterface methods (this is one of the reasons why we use interfaces at all).

There are some solutions that come in mind.

1) Just leave it as is, but rename public methods that are not in interfaces, so it work as warning for those alien client code implementors, for example, rename methodToSetSomethingFromFriendClass to internalUseOnly_methodToSetSomethingFromFriendClass. Reduces risk of occassional usage, but doesn't prohibits calling methods technically.

2) Using adaptor (decorator) pattern. Pass to an external code only decorated instance which doesn't has methodToSetSomethingFromFriendClass method. It does solve problem of unwanted method access but complicates another parts of our code very much. It seems there is no overall profit.

3) Not really checked yet vialibity of this idea: utilize the following fact. Base class can declare protected method so all friend classes we control may be extended from that base class keeping our methodToSetSomethingFromFriendClass mehod marked as protected being effectively protected from external code, but callable from "friend" class. Technically this allows protection, but requires friend classes to inherit from common base class, drawbacks of such are well known.

Do you know anything better? Links to the articles are appreciated as well as sharing experience on this topic research.