ISP in PHP — PSR interfaces as good examples

<?php
declare(strict_types=1);

// PSR-11: ContainerInterface — the minimal contract for a DI container
// Only two methods because that's all a caller ever needs
use Psr\Container\ContainerInterface;

// PSR-11 interface (shown for study):
// interface ContainerInterface {
//     public function get(string $id): mixed;
//     public function has(string $id): bool;
// }

// Any class that only READS from the container gets ContainerInterface
final class ViewFactory
{
    public function __construct(private readonly ContainerInterface $container) {}

    public function make(string $viewClass): View
    {
        if (!$this->container->has($viewClass)) {
            throw new \InvalidArgumentException("View not registered: {$viewClass}");
        }
        return $this->container->get($viewClass);
    }
}

// PSR-3: LoggerInterface — segregated by log level
// (simplified — shows the ISP thinking behind it)
use Psr\Log\LoggerInterface;

// Logger is injected only into classes that need logging
// They don't get the whole container — just the logger interface
final class PaymentProcessor
{
    public function __construct(
        private readonly PaymentGateway  $gateway,
        private readonly LoggerInterface $logger,  // PSR-3 — just logging, nothing else
    ) {}

    public function charge(Order $order): PaymentResult
    {
        try {
            $result = $this->gateway->charge($order);
            $this->logger->info('Payment succeeded', ['order_id' => $order->id]);
            return $result;
        } catch (PaymentException $e) {
            $this->logger->error('Payment failed', [
                'order_id' => $order->id,
                'error'    => $e->getMessage(),
            ]);
            throw $e;
        }
    }
}

// Your own application following PSR ISP lessons

// Segregated notification interfaces — each describes one delivery channel
interface EmailChannel
{
    public function sendEmail(User $user, string $subject, string $body): void;
}

interface SmsChannel
{
    public function sendSms(User $user, string $message): void;
}

interface PushChannel
{
    public function sendPush(User $user, string $title, string $body): void;
}

// Implementation can combine all channels
final class NotificationDriver implements EmailChannel, SmsChannel, PushChannel
{
    public function sendEmail(User $user, string $subject, string $body): void
    {
        Mail::to($user->email)->send(new GenericMail($subject, $body));
    }

    public function sendSms(User $user, string $message): void
    {
        Twilio::message($user->phone, $message);
    }

    public function sendPush(User $user, string $title, string $body): void
    {
        FCM::push($user->push_token, $title, $body);
    }
}

// Password reset service — only needs email, not SMS or push
final class PasswordResetNotifier
{
    public function __construct(private readonly EmailChannel $email) {}

    public function notify(User $user, string $resetUrl): void
    {
        $this->email->sendEmail(
            $user,
            'Reset your password',
            "Click here to reset: {$resetUrl}"
        );
    }
}

// Order shipped — needs both email AND push, but not SMS
final class ShipmentNotifier
{
    public function __construct(
        private readonly EmailChannel $email,
        private readonly PushChannel  $push,
    ) {}

    public function notify(User $user, Order $order): void
    {
        $this->email->sendEmail($user, 'Your order shipped', "Order #{$order->id} is on its way!");
        $this->push->sendPush($user, 'Order shipped!', "Order #{$order->id}");
    }
}