<?php

abstract class Animal {

    abstract public function getNoise() : string;

}

class Dog extends Animal {

    public function getNoise() : string { return "woof"; }

}

class Cat extends Animal {

    public function getNoise() : string { return "miaow"; }

}

​

/**

 * @template-covariant T

 */

class Collection {

    /** @var array<int, T> */

    public array $list = [];

}

​

/**

 * @param Collection<Animal> $collection

 */

function getNoises(Collection $collection) : void {

    foreach ($collection->list as $animal) {

        echo $animal->getNoise();

    }

}

​

/**

 * @param Collection<Dog> $dog_collection

 */

function takesDogList(Collection $dog_collection) : void {

    getNoises($dog_collection);

}