[RFC] Properties in interfaces

Hi, [Proposal] Allow interfaces to define properties. The implementing class must have the same properties with the same name/type. As the goal of the interfaces is to define the public "surface" of the implementing class, I think that the public properties are also part of that. [Use case] - tl;dr; Right now I’m working on a product configurator. The input I get, I turn them into ValueObjects in the initial phase of the request. Then later based on some conditions, I turn them in a deterministic but very varying order into another ValueObject which has the same properties, but all of them are readonly. I tried to create somehow the same: https://3v4l.org/Vb7fE#v8.2.4 There are places where I have an array with a mix of them and I want to access the $id or $axisOrientation. I also have methods on them. I know an abstract class could do this for me but I feel that this would be the most accurate way to define what I want. [Example] ```php interface Vehicle { public int $serialNumber; public function drive(): void; } /* * This would lead to an error, something like: * Class Car contains 1 abstract property and must therefore be * declared abstract or implement the remaining property * (Vehicle::$serialNumber) */ class Car implements Vehicle { public function drive() : void { } } // This would lead to an error too, because the type is different. class Bike implements Vehicle { public string $serialNumber; public function drive() : void { } } // This would lead to an error too. The rule for readonly properties // would be the same as right now between normal classes. class Bike implements Vehicle { public readonly string $serialNumber; public function drive() : void { } } ``` [Implementation] Personally I don’t have experience in php internal development. I do have C++ / some C experience. I'm going through the PHP Internals book and every possible tutorial I find. I definitely would like to take part in it, and I'm planning to do more later. -- Bests Zoltán Fekete