classes.php

<?php

class Liger implements Animal
{
    public function getSpecies(): LionTiger
    {
        return new LionTiger();
    }
}

class LionTiger implements Species
{
    function getName()
    {
        return 'Lion & Tiger';
    }
}

interfaces.php

<?php

interface Species
{
    function getName(): string;
}

interface Animal
{
    function getSpecies(): Species;
}