The Chain of Responsibility 

Let's create a PHP example:

The first thing I want to do is to create is the interface Handler. We add the two method definitions:

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

The method stub setNext Next we create a class called AbstractHandler it implements Handler and missing methods setNext and handle
we add code to setNext and to this interpase

$this->nextHandler = $handler;
return $handler;

and

if ($this->nextHandler) {
    return $this->nextHandler->handle($request);
}

return null;

we next add the class SquirrelHandler which extends AbstractHandler to it we add:

public function handle(string $request): ?string
{
    if ($request === "Nut") {
        return "Squirrel: I'll eat the " . $request . ".
";
    } else {
        return parent::handle($request);
    }
}

let's add another anamale DogHandler

public function handle(string $request): ?string
{
   if ($request === "MeatBall") {
       return "Dog: I'll eat the " . $request . ".\n";
   } else {
       return parent::handle($request);
   }
}

lastly we add the class MouseHandler and have it extends AbstractHandler and we add the code:

public function handle(string $request): ?string
{
   if ($request === "Cheese") {
       return "Mouse: I'll eat the " . $request . ".
";
   } else {
      return parent::handle($request);
   }
}

for our demo we go to the index.php file and add sime includes:

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

and we also need to add the folloing includes:

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

now to the file we add the client function:

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

next we define the following local varables:

$mouse = new MouseHandler;
$squirrel = new SquirrelHandler;
$dog = new DogHandler;

next we consider the following code to demo the chain of responsibity

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

echo "Chain: Mouse > Squirrel > Dog

";
clientCode($mouse);
echo "\n";

echo "Subchain: Squirrel > Dog

";
clientCode($squirrel);

thanks for being a part of The Ray Code. Be good...