Polimorfismo de sobrecarga

<!DOCTYPE html>
<html>
<head>
    <title>Sobrecarga</title>
</head>
<body>
    <h1>
<?php
require_once 'Mamifero.php';
require_once 'Lobo.php';

$c = new Lobo();
$c->emitirSom();
echo "<hr>";

?>
    </h1>
</body>
</html>

// classe Animal
<?php

abstract class Animal{
    protected $peso;
    protected $idade;
    protected $membros;
    public abstract function emitirSom();
}

// classe Mamifero
<?php
require_once 'Animal.php';
class Mamifero extends Animal{
   protected $corPelo;
   public function emitirSom(){
    echo "<p>Som mamifero</p>";
   }
}

// classe lobo

</<?php
require_once 'Mamifero.php';
class Lobo extends Mamifero{

}

//classe cachorro

<?php

require_once 'Lobo.php';
class Cachorro extends Lobo{

}