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

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.

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

<?phpclass 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.

Does not compute. We had the same discussion yesterday:
Can parameter types be specialized in PHP

All your derived classes must implement the method signatures identically.

This is something that would ideally be checked at runtime. But in PHP the parser does. (To compensate, PHP does not check private/protected attribute access at parsing time, but let that one rather blow up at runtime.)

If you want to enforce a more stringent type, I would advise:

 assert( is_a($Object, "AClass") );