What are alternatives to variance and invariance and how do they compare?

In programming, especially in object-oriented languages, variance and invariance determine how subtypes relate to their parent types. Alternatives include contravariance and covariance.

variance, invariance, contravariance, covariance, object-oriented programming, type safety


        <?php
        // Example of Covariance
        class Animal {
            public function makeSound() {
                return "Some sound";
            }
        }

        class Dog extends Animal {
            public function makeSound() {
                return "Bark";
            }
        }

        function getAnimal(): Animal {
            return new Dog(); // Covariant return type
        }

        $animal = getAnimal();
        echo $animal->makeSound(); // Outputs: Bark
        ?>