Declaration not compatible with interface (PHP)

Question

Let's say I have one parent class with two child classes like this

abstract class Vehicle {
    public function setManufacturer(string $manufacturer) { ... }
}

class Bicycle extends Vehicle {
    public function addSaddle() { ... }
}

class Car extends Vehicle {
    public function setSpeedLimit(float $limit) { ... }
}

Now I want to use these objects with an interface like this

interface VehicleInterface {
    public function create(Vehicle $vehicle);
}

class BicycleService implements VehicleInterface {
    public function create(Bicycle $bicycle) {
        $bicycle->setManufacturer('Some company'); // Common for all Vehicle objects
        $bicycle->addSaddle(); // Common only for Bicycle objects
    }
}

It seems that this is not possible since BicycleService create(...) is not compatible with the interface. Is there any way around this other than removing type hints?

Answer 1

Consider this:

class NotBicycle implements Vehicle { ... }

interface VehicleInterface {
    public function create(Vehicle $vehicle);
}

class BicycleService implements VehicleInterface {
    public function create(Bicycle $bicycle) {
        $bicycle->setManufacturer('Some company'); // Common for all Vehicle objects
        $bicycle->addSaddle(); // Common only for Bicycle objects
    }
}

function createVehicle(VehicleInterface $service, Vehicle $vehicle) {
    $service->create($vehicle);
}
$service = new BicycleService();
$vehicle = new NotBicycle();
createVehicle($service, $vehicle);

Even if you were somehow able to only accept a Bicycle in the BicycleService, createVehicle($service, $vehicle) will still work because VehicleInterface has a method create(Vehicle $vehicle). Therefore in order to get this to work the way you want it, you need to basically break what an interface is.

Your only real option is to add a runtime type check. Like e.g.

class BicycleService implements VehicleInterface {
    public function create(Vehicle $bicycle) {
        if (!$bicycle instance of Bicycle) { 
           throw new TypeError('Expected Bicycle but got '.get_class($bicycle)); 
        }
        $bicycle->setManufacturer('Some company'); // Common for all Vehicle objects
        $bicycle->addSaddle(); // Common only for Bicycle objects
    }
}

What you are trying to do is called a covariant method parameter type and is not allowed in most object-oriented programming languages because it breaks the Liskov substitution principle

Read more on parameter covariance and contravariance