Private method overriding and visibility

Question

I'm having a hard time trying to understand the output of the following code:

class Bar 
{
    public function test() {
        $this->testPublic();
        $this->testPrivate();
    }

    public function testPublic() {
        echo "Bar::testPublic\n";
    }

    private function testPrivate() {
        echo "Bar::testPrivate\n";
    }
}

class Foo extends Bar 
{
    public function testPublic() {
        echo "Foo::testPublic\n";
    }

    private function testPrivate() {
        echo "Foo::testPrivate\n";
    }
}

$myFoo = new foo();
$myFoo->test();

Output:

Foo::testPublic
Bar::testPrivate 

Class Foo overrides testPublic() and testPrivate(), and inherits test(). When I call test(), there is an explicit instruction envolving $this pseudo variable, so after I created $myFoo instance, the final calls of test() function would be $myFoo->testPublic() and $myFoo->testPrivate(). The first output is as I expected, since I overrode testPublic() method to echo Foo::testPublic. But the second output makes no sense to me. Why is it Bar::testPrivate if I overrode testPrivate() method? Also the private method from parent class wouldn't be inherited anyway, by definition! It makes no sense. Why is the parent method the one being called???

Answer 1

The problem with your code is that the method Bar::testPrivate is private, therefore it cannot be overridden by child classes. For starters, I recommend that you read up on visibility in PHP - http://www.php.net/manual/en/language.oop5.visibility.php. There you will learn that only public and protected class member methods/properties can be overridden, private ones cannot.

As a good example, try changing the visibility of the Bar::testPrivate method to either public or protected, without altering anything else in your example code. Now try and run your tests. What happens? This:

PHP Fatal error: Access level to Foo::testPrivate() must be protected (as in class Bar) or weaker

The big question is: "why?". Well, you have now overridden Bar::testPrivate with a private Foo:testPrivate. This new private method is out of scope for Bar::test, because private class members are visible to their current class only, NOT the parent/child classes!

Therefore, as you can see, OOP provides a certain amount of encapsulation for class members, and it can be quite confusing if you don't take the time to understand it.