1.php

<?php

class User
{
    private $name;

    public function setName($name)
    {
        $this->name = $name;
    }

    public function getName()
    {
        return $this->name;
    }
}

class Employee extends User
{
    public $role = 'employee';
}

class Catalog
{
    private $users = [];

    public function addUser(User $user)
    {
        $this->users[] = $user;
    }

    public function getNames()
    {
        foreach ($this->users as $user) {
            echo $user->getName() . PHP_EOL;
        }
    }
}


$user = new User;
$user->setName('Vladimir Guts');

$user2 = new Employee;
$user2->setName('Dmitry Golub');
$user2->role = 'manager';

$catalog = new Catalog;
$catalog->addUser($user);
$catalog->addUser($user2);
$catalog->getNames();

2.php

<?php

class Notifier
{
    private $channel = 'slack';

    public function notify($notification)
    {
        if ($this->channel == 'slack') {
            $this->sendViaSlack($notification);
        } elseif ($this->channel == 'email') {
            $this->sendViaEmail($notification);
        }
    }

    private function sendViaSlack($notification) {}
    private function sendViaEmail($notification) {}
}



$notification = new Notifier();

$notification->notify("Hello");

3.php

<?php

class Auth
{
    public function login($username, $password)
    {
        $user = $this->getUserData($username, $password);
        if ($user) {
            $this->auth($user);
        }
    }

    public function getUserData($user, $password)
    {
        $dbUser = $this->db("SELECT * FROM users WHERE username = '" . $user . "'");
        if ($dbUser['password'] == $password) {
            return $dbUser;
        }
    }

    public function auth($user)
    {
        $_SESSION['auth'] = $user;
        $this->log('User was logged in');
    }


}