Covariance and Contravariance

In PHP 7.2.0, partial contravariance was introduced by removing type restrictions on parameters in a child method. As of PHP 7.4.0, full covariance and contravariance support was added.

Covariance allows a child's method to return a more specific type than the return type of its parent's method. Whereas, contravariance allows a parameter type to be less specific in a child method, than that of its parent.

A type declaration is considered more specific in the following case:

A type class is considered less specific if the opposite is true.

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();

The above example will output:

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);

The above example will output:

Ricky eats AnimalFood
Mavrick eats Food

But what happens if $kitty tries to eat the $banana?

$kitty->eat($banana);

The above example will output:

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