Override method parameter with child interface as a new parameter

oop, php

Solution

It does break SOLID rules. You declare `Car::setEngine` to accept one parameter of type `Engine`, but the child `WaterCar::setEngine` accepts a parameter of type `HydroEngine`. Even if `HydroEngine` is a subtype of `Engine`, it's still a different type.

When a class `Foo implements WaterCar`, it is also true that this class is an `instanceof Car`. But `Foo::setEngine` accepts a `HydroEngine`, but does not accept an `Engine`. So `Foo::setEngine` supposedly `implements Car`, yet does not accept a parameter of type `Engine`. Which breaks the Liskov substitution principle. You cannot change the type of parameters in subclassed interfaces, period.

The keyword for inheritance is explicitly `extends`. A subclass does exactly the same as the parent class and possibly more. It cannot do less than the parent. Since `HydroEngine` is a specialized subtype of `Engine`, this would mean a `WaterCar` does less than a `Car`, since it only accepts a more narrow subtype of `Engine`. E.g.:

function (Car $car) {
    $engine = new EngineImplementation;
    $car->setEngine($engine);
}

The above code would implode if you passed in a `WaterCar`, because it does not accept an `Engine`.

Problem

I cant figure out why this code does not work in PHP? ``` <?php interface Engine { function run(); } interface HydroEngine extends Engine { function run(); } interface Car { function setEngine(Engine $engine); } interface WaterCar extends Car { function setEngine(HydroEngine $engine); } ?> ``` It seems it does not break any OOP rules, but why it gives me an error? `Fatal error: Declaration of WaterCar::setEngine() must be compatible with Car::setEngine(Engine $engine)`

Original source