Covariance

To illustrate how covariance works, a simple abstract parent class, Animal is created. Animal will be extended by children classes, Cat, and Dog.

<?php

abstract class Animal
{
    protected string $name;

    public function __construct(string $name)
    {
        $this->name = $name;
    }

    abstract public function speak();
}

class Dog extends Animal
{
    public function speak()
    {
        echo $this->name . " barks";
    }
}

class Cat extends Animal 
{
    public function speak()
    {
        echo $this->name . " meows";
    }
}

Note that there aren't any methods which return values in this example. A few factories will be added which return a new object of class type Animal, Cat, or Dog.

<?php

interface AnimalShelter
{
    public function adopt(string $name): Animal;
}

class CatShelter implements AnimalShelter
{
    public function adopt(string $name): Cat // instead of returning class type Animal, it can return class type Cat
    {
        return new Cat($name);
    }
}

class DogShelter implements AnimalShelter
{
    public function adopt(string $name): Dog // instead of returning class type Animal, it can return class type Dog
    {
        return new Dog($name);
    }
}

$kitty = (new CatShelter)->adopt("Ricky");
$kitty->speak();
echo "\n";

$doggy = (new DogShelter)->adopt("Mavrick");
$doggy->speak();

Ricky meows
Mavrick barks

Contravariance

Continuing with the previous example with the classes Animal, Cat, and Dog, a class called Food and AnimalFood will be included, and a method eat(AnimalFood $food) is added to the Animal abstract class.

<?php

class Food {}

class AnimalFood extends Food {}

abstract class Animal
{
    protected string $name;

    public function __construct(string $name)
    {
        $this->name = $name;
    }

    public function eat(AnimalFood $food)
    {
        echo $this->name . " eats " . get_class($food);
    }
}

In order to see the behavior of contravariance, the eat method is overridden in the Dog class to allow any Food type object. The Cat class remains unchanged.

<?php

class Dog extends Animal
{
    public function eat(Food $food) {
        echo $this->name . " eats " . get_class($food);
    }
}

The next example will show the behavior of contravariance.

<?php

$kitty = (new CatShelter)->adopt("Ricky");
$catFood = new AnimalFood();
$kitty->eat($catFood);
echo "\n";

$doggy = (new DogShelter)->adopt("Mavrick");
$banana = new Food();
$doggy->eat($banana);

Ricky eats AnimalFood
Mavrick eats Food

$kitty->eat($banana);

Fatal error: Uncaught TypeError: Argument 1 passed to Animal::eat() must be an instance of AnimalFood, instance of Food given

Property variance

By default, properties are neither covariant nor contravariant, hence invariant. That is, their type may not change in a child class at all. The reason for that is "get" operations must be covariant, and "set" operations must be contravariant. The only way for a property to satisfy both requirements is to be invariant.

As of PHP 8.4.0, with the addition of abstract properties (on an interface or abstract class) and virtual properties, it is possible to declare a property that has only a get or set operation. As a result, abstract properties or virtual properties that have only a "get" operation required may be covariant. Similarly, an abstract property or virtual property that has only a "set" operation required may be contravariant.

Once a property has both a get and set operation, however, it is no longer covariant or contravariant for further extension. That is, it is now invariant.

<?php
class Animal {}
class Dog extends Animal {}
class Poodle extends Dog {}

interface PetOwner
{
    // Only a get operation is required, so this may be covariant.
    public Animal $pet { get; }
}

class DogOwner implements PetOwner
{
    // This may be a more restrictive type since the "get" side
    // still returns an Animal.  However, as a native property
    // children of this class may not change the type anymore.
    public Dog $pet;
}

class PoodleOwner extends DogOwner
{
    // This is NOT ALLOWED, because DogOwner::$pet has both
    // get and set operations defined and required.
    public Poodle $pet;
}
?>