Chain-of-responsibility pattern php

We start by first creating an interface called Handler.

interface Handler
{
    public function setNext(Handler $handler): Handler;
    public function handle(string $request): ?string;
}

Next we will create an AbstractHandler which will be extened with the Handler interface.

class AbstractHandler implements Handler
{
    public function setNext(Handler $handler): Handler
    {
        $this->nextHandler = $handler;
        return $handler;
    }
    public function handle(string $request): ?string
    {
        if ($this->nextHandler) {
            return $this->nextHandler->handle($request);
        }
        return null;
    }
}

Let's create a menagerie of animals a cat, dog and a mouse. Each would like a treat so let code a senairo. Each will be extended with the AbstractHandler. For dog we have the DogHandler.

class DogHandler extends AbstractHandler
{
    public function handle(string $request): ?string
    {
        if ($request === "Bone") {
            return "Dog: I'll eat the " . $request . ".<br/>";
        } else {
            return parent::handle($request);
        }
    }
}

For the cat let's create a CatHandler.

class CatHandler extends AbstractHandler
{
    public function handle(string $request): ?string
    {
        if ($request === "Catnip") {
            return "Cat: I'll eat the " . $request . ".<br/>";
        } else {
            return parent::handle($request);
        }
    }
}

The last animal in our menagerie will be a mouse which is the MouseHandler.

class MouseHandler extends AbstractHandler
{
    public function handle(string $request): ?string
    {
        if ($request === "Cheese") {
            return "Mouse: I'll eat the " . $request . ".<br/>";
        } else {
            return parent::handle($request);
        }
    }
}

Let's put this altogether in the index.php
We first add the includes we need

include_once ('Handler.php');
include_once ('AbstractHandler.php');

include_once('CatHandler.php');
include_once('MouseHandler.php');
include_once ('DogHandler.php');

Let's add a client function

function clientCode(Handler $handler)
{
    foreach (["Catnip", "Bone", "Cup of coffee"] as $food) {
        echo "Client: Who wants a " . $food . "?<br/>";
        $result = $handler->handle($food);
        if ($result) {
            echo "  " . $result;
        } else {
            echo "  " . $food . " was left untouched.<br/>";
        }
    }
}

Now for the play/demo we put it altogether in the following statments.

$cat = new CatHandler;
$mouse = new MouseHandler;
$dog = new DogHandler;

$cat->setNext($mouse)->setNext($dog);

echo "Chain: Mouse > Cat > Dog<br/>";
clientCode($cat);
echo "<br/>";

echo "Subchain: Mouse > Dog<br/>";
clientCode($mouse);

Let's now look at this through a browser. You should have:

Chain: Mouse > Cat > Dog
Client: Who wants a Catnip?
Cat: I'll eat the Catnip.
Client: Who wants a Bone?
Dog: I'll eat the Bone.
Client: Who wants a Cup of coffee?
Cup of coffee was left untouched.

Subchain: Mouse > Dog
Client: Who wants a Catnip?
Catnip was left untouched.
Client: Who wants a Bone?
Dog: I'll eat the Bone.
Client: Who wants a Cup of coffee?
Cup of coffee was left untouched.

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