How to implement the Mediator design pattern in PHP for communication between objects?

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interface Mediator {
    public function sendMessage(string $message, Colleague $colleague);
}

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interface Colleague {
    public function setMediator(Mediator $mediator);
    public function sendMessage(string $message);
    public function receiveMessage(string $message);
}

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class ConcreteMediator implements Mediator {
    private $colleagues = [];

    public function addColleague(Colleague $colleague) {
        $this->colleagues[] = $colleague;
    }

    public function sendMessage(string $message, Colleague $colleague) {
        foreach($this->colleagues as $col) {
            if($col != $colleague) {
                $col->receiveMessage($message);
            }
        }
    }
}

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class ConcreteColleague implements Colleague {
    private $mediator;

    public function setMediator(Mediator $mediator) {
        $this->mediator = $mediator;
    }

    public function sendMessage(string $message) {
        $this->mediator->sendMessage($message, $this);
    }

    public function receiveMessage(string $message) {
        echo "Received message: " . $message . "
";
    }
}

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$mediator = new ConcreteMediator();

$colleague1 = new ConcreteColleague();
$colleague2 = new ConcreteColleague();
$colleague3 = new ConcreteColleague();

$colleague1->setMediator($mediator);
$colleague2->setMediator($mediator);
$colleague3->setMediator($mediator);

$mediator->addColleague($colleague1);
$mediator->addColleague($colleague2);
$mediator->addColleague($colleague3);

$colleague1->sendMessage("Hello, colleagues!");
$colleague2->sendMessage("Hi, colleague 1!");
$colleague3->sendMessage("Hey, everyone!");

// Output:
// Received message: Hello, colleagues!
// Received message: Hi, colleague 1!
// Received message: Hey, everyone!