Why Can't I Override Method Parameters with a Child Interface as a New Parameter in PHP?

Overriding Method Parameters with Child Interface as a New Parameter: Why It Fails

In PHP, method overriding allows a subclass to redefine the behavior of a method inherited from the parent class. However, there are certain restrictions in place to ensure type compatibility and adherence to OOP principles.

Consider the following code snippet, which attempts to override the setEngine() method in the WaterCar interface with a new parameter of type HydroEngine:

<code class="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);
}

?></code>

This code snippet fails with the following error:

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

The reason for this error is that overriding method parameters violates the Liskov substitution principle. The WaterCar interface declares a different parameter type for its setEngine() method than the Car interface.

Specifically, the Car interface expects a parameter of type Engine, while the WaterCar interface expects a parameter of type HydroEngine. Even though HydroEngine is a subclass of Engine, it is still considered a different type.

As a result, a class that implements WaterCar may not be able to fully implement Car because it cannot accept an Engine parameter. This breaks the Liskov substitution principle, which states that a subtype should be able to be used in place of its parent type without causing unexpected behavior.

In other words, if a function expects a Car, it should be able to accept a WaterCar without any issues. However, in this case, the function would fail if passed a WaterCar because it is expecting an Engine parameter.

To resolve this issue, it is necessary to maintain compatibility in method parameters between parent and child interfaces. If special behaviors are required for certain subclasses, it is better to define additional methods instead of overriding parameter types.

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