Can parameter types be specialized in PHP

Question

Say we've got the following two classes:

abstract class Foo {
    public abstract function run(TypeA $object);
}

class Bar extends Foo {
    public function run(TypeB $object) {
        // Some code here
    }
}

The class TypeB extends the class TypeA.

Trying to use this yields the following error message:

Declaration of Bar::run() must be compatible with that of Foo::run()

Is PHP really this broken when it comes to parameter types, or am I just missing the point here?

Answer 1

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This answer is outdated since PHP 7.4 (partially since 7.2).

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The behavior you describe is called covariance and is simply not supported in PHP. I don't know the internals but I might suspect that PHP's core does not evaluate the inheritance tree at all when applying the so called "type hint" checks.

By the way, PHP also doesn't support contravariance on those type-hints (a feature commonly support in other OOP languages) - most likely to the reason is suspected above. So this doesn't work either:

abstract class Foo {
    public abstract function run(TypeB $object);
}

class Bar extends Foo {
    public function run(TypeA $object) {
        // Some code here
    }
}

And finally some more info: http://www.php.net/~derick/meeting-notes.html#implement-inheritance-rules-for-type-hints

Answer 2

This seems pretty consistent with most OO principals. PHP isn't like .Net - it doesn't allow you to override class members. Any extension of Foo should slide into where Foo was previously being used, which means you can't loosen constraints.

The simple solution is obviously to remove the type constraint, but if Bar::run() needs a different argument type, then it's really a different function and should ideally have a different name.

If TypeA and TypeB have anything in common, move the common elements to a base class and use that as your argument constraint.