Why Can't I Override Method Parameters in Child Interfaces in PHP?

Overriding Method Parameters: Pitfalls and Principles

In object-oriented programming, inheritance allows derived classes to extend or modify the behavior of their parent classes. However, there are certain limitations when it comes to overriding method parameters.

Consider the following code, where we have an interface Car with a method setEngine that accepts a parameter of type Engine. We also have a child interface WaterCar that extends Car and declares a different method setEngine, which accepts a parameter of type HydroEngine, a subtype of Engine.

<code class="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);
}</code>

This code raises an error:

Fatal error: Declaration of WaterCar::setEngine() must be compatible with Car::setEngine(Engine $engine)

Reasoning:

The reason for this error is that WaterCar violates the Liskov substitution principle. This principle states that "subclasses must be substitutable for their base classes." In this case, a WaterCar instance cannot be substituted for a Car instance because its setEngine method has a different parameter type.

Even though HydroEngine is a subtype of Engine, it is still a different type. By overriding the method parameters in the child interface, WaterCar breaks the compatibility with its parent interface.

This can lead to runtime errors if code that expects a Car instance is passed a WaterCar instance. For example, the following code:

<code class="php">function checkEngine(Car $car) {
    $engine = new EngineImplementation();
    $car->setEngine($engine);
}</code>

Would fail if called with a WaterCar instance because it cannot accept an Engine parameter.

Conclusion:

When overriding methods in derived classes, it is essential to maintain compatibility with the parent class by preserving the same parameter types. Otherwise, it can lead to runtime errors and violate object-oriented design principles.

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