index.php

spl_autoload_register(function($class) {
    // echo 'register class:'.$class."<br>";
    require_once("classes/{$class}.php");
});

// echo 'hey there on line 7'."<br>";

$cat = new Cat;
$dog = new Dog;
$tortoise = new Tortoise;

echo $cat->talk();
echo $dog->talk();
echo $tortoise->talk();

classes/Talkative.php

interface Talkative {
    public function talk();
}

classes/Cat.php

class Cat implements Talkative {
    public function talk() {
        return 'Meow' . '<br>';
    }
}

classes/Dog.php

class Dog implements Talkative {
    public function talk() {
        return 'Woof' . '<br>';
    }
}

classes/Tortoise.php

class Tortoise implements Talkative {
    public function talk() {
        return 'Yak yak yak yak ...' . '<br>';
    }
}

Filenames and class names must be THE SAME