Php type hinting not getting along with interfaces and abstract classes?

Question

I think it'll be much easier to see the problem in a code example than writing the question in the first place. Here is my php code:

<?php

interface AnInterface
{
        public function method();
}    

class AClass implements AnInterface
{
        public function method()
        {
                echo __METHOD__;
        }
}    

abstract class AnAbstractClass
{
        abstract public function method( AnInterface $Object );
}

class ConcreteClass extends AnAbstractClass
{
        public function method( AClass $Object )
        {
                $Object->method();
        }
}

$Object1 = new ConcreteClass();
$Object2 = new AClass();

$Object1->method( $Object2 );

The above code causes the following error:

Fatal error: Declaration of ConcreteClass::method() must be compatible with that of AnAbstractClass::method()

The problem is that php doesn't seem to be recognizing the signatures of AnAbstractClass::method and ConcreteClass::method as compatible. Am I doing something wrong? Thanks!

Answer 1

php doesn't seem to be recognizing the signatures of AnAbstractClass::method and ConcreteClass::method as compatible.

PHP is correct, they're not compatible. By allowing only instances of AClass (or its children) to be passed to ConcreteClass::method, you're breaking the contract that AnAbstractClass provides: Any of its subclasses must accept AnInterface as an argument to its method().

If your example worked, and I had another class BClass implementing AnInterface, we'd have a situation where according to AnAbstractClass, method() should accept instances of BClass, while according to ConcreteClass, it shouldn't.

Change your signature for ConcreteClass::method to match that of AnAbstractClass::method.

Answer 2

Here's an example that shows, why this is not allowed:

<?php
class BClass implements AnInterface { }

function moo(AnAbstractClass $abstract)
{
    $b = new BClass();
    $abstract->method($b);
}

This would be a valid code, but it would fail, if you pass a ConcreteClass to moo, because its method ConcreteClass::method does not allow BClass.

It is complicated, but it is easier to understand, if you see an example.