Lab 07: Design Patterns

Objective

Implement the most commonly used design patterns in PHP: Singleton for shared resources, Factory/Abstract Factory for object creation, Observer for event systems, Strategy for swappable algorithms, Decorator for behaviour composition, and Command for encapsulating actions.

Background

Design patterns are reusable solutions to recurring problems. In PHP they appear everywhere: Laravel's IoC container (Singleton + Factory), Symfony's EventDispatcher (Observer), payment gateways (Strategy), middleware stacks (Decorator), and command buses (Command). Learning to recognise and implement patterns makes framework internals transparent.

Time

35 minutes

Prerequisites

• PHP Practitioner Lab 01 (Traits & Interfaces)

Tools

• Docker: zchencow/innozverse-php:latest

---

Lab Instructions

Step 1: Singleton & Factory

docker run --rm zchencow/innozverse-php:latest php -r '
<?php
// ── Singleton ────────────────────────────────────────────────────────────────
// Ensures only ONE instance exists — used for DB connections, config, loggers
class DatabaseConnection {
    private static ?self $instance = null;
    private int $queryCount = 0;
    private array $log = [];

    // Private constructor prevents "new DatabaseConnection()"
    private function __construct(private string $dsn) {
        $this->log[] = "Connected to: {$dsn}";
    }

    public static function getInstance(string $dsn = "sqlite::memory:"): self {
        if (self::$instance === null) {
            self::$instance = new self($dsn);
        }
        return self::$instance;
    }

    public function query(string $sql): string {
        $this->queryCount++;
        $this->log[] = "Query #{$this->queryCount}: {$sql}";
        return "Result of: {$sql}";
    }

    public function stats(): array {
        return ["queries" => $this->queryCount, "dsn" => $this->dsn];
    }

    // Prevent cloning and unserialization
    private function __clone() {}
    public function __wakeup(): never {
        throw new \Exception("Cannot unserialize singleton");
    }
}

echo "=== Singleton ===" . PHP_EOL;
$db1 = DatabaseConnection::getInstance();
$db2 = DatabaseConnection::getInstance();
$db1->query("SELECT * FROM products");
$db2->query("SELECT * FROM orders");   // same instance!
echo "Same instance: " . ($db1 === $db2 ? "yes" : "no") . PHP_EOL;
echo "Queries: " . $db1->stats()["queries"] . PHP_EOL;

// ── Factory Method ────────────────────────────────────────────────────────────
echo PHP_EOL . "=== Factory Method ===" . PHP_EOL;

interface PaymentGateway {
    public function charge(float $amount): array;
    public function getName(): string;
}

class StripeGateway implements PaymentGateway {
    public function charge(float $amount): array {
        return ["gateway" => "Stripe", "amount" => $amount, "fee" => round($amount*0.029+0.30,2), "status" => "ok"];
    }
    public function getName(): string { return "Stripe"; }
}

class PayPalGateway implements PaymentGateway {
    public function charge(float $amount): array {
        return ["gateway" => "PayPal", "amount" => $amount, "fee" => round($amount*0.034+0.30,2), "status" => "ok"];
    }
    public function getName(): string { return "PayPal"; }
}

class CryptoGateway implements PaymentGateway {
    public function charge(float $amount): array {
        return ["gateway" => "Crypto", "amount" => $amount, "fee" => round($amount*0.001,4), "status" => "ok"];
    }
    public function getName(): string { return "Crypto"; }
}

// Factory: creates the right object based on config/input
class PaymentGatewayFactory {
    public static function create(string $type): PaymentGateway {
        return match($type) {
            "stripe" => new StripeGateway(),
            "paypal" => new PayPalGateway(),
            "crypto" => new CryptoGateway(),
            default  => throw new \InvalidArgumentException("Unknown gateway: {$type}"),
        };
    }
}

$gateways = ["stripe", "paypal", "crypto"];
$amount   = 864.00;
foreach ($gateways as $type) {
    $gw     = PaymentGatewayFactory::create($type);
    $result = $gw->charge($amount);
    printf("  %-8s fee=\$%-7.4f net=\$%.2f%s",
        $result["gateway"], $result["fee"], $amount - $result["fee"], PHP_EOL);
}
'

---

Step 2: Observer, Strategy, Decorator, Command

docker run --rm zchencow/innozverse-php:latest php -r '
<?php
// ── Observer / Event System ───────────────────────────────────────────────────
echo "=== Observer (Event System) ===" . PHP_EOL;

interface EventListener {
    public function handle(string $event, array $data): void;
}

class EventEmitter {
    private array $listeners = [];

    public function on(string $event, EventListener|callable $listener): void {
        $this->listeners[$event][] = $listener;
    }

    public function emit(string $event, array $data = []): void {
        foreach ($this->listeners[$event] ?? [] as $listener) {
            is_callable($listener) ? $listener($data) : $listener->handle($event, $data);
        }
    }
}

class EmailNotifier implements EventListener {
    public array $sent = [];
    public function handle(string $event, array $data): void {
        $this->sent[] = "Email: [{$event}] Order #{$data["orderId"]} - {$data["product"]}";
    }
}

class InventoryManager implements EventListener {
    public array $changes = [];
    public function handle(string $event, array $data): void {
        if ($event === "order.placed") {
            $this->changes[] = "Deduct stock: {$data["product"]} × {$data["qty"]}";
        }
    }
}

$events    = new EventEmitter();
$email     = new EmailNotifier();
$inventory = new InventoryManager();

$events->on("order.placed",    $email);
$events->on("order.placed",    $inventory);
$events->on("order.shipped",   $email);
$events->on("order.cancelled", $email);
$events->on("order.cancelled", fn($d) => print("  ⚡ Webhook: refund for #{$d["orderId"]}" . PHP_EOL));

$events->emit("order.placed",    ["orderId" => 1001, "product" => "Surface Pro",  "qty" => 2]);
$events->emit("order.shipped",   ["orderId" => 1001, "product" => "Surface Pro",  "qty" => 2]);
$events->emit("order.cancelled", ["orderId" => 1002, "product" => "USB-C Hub",    "qty" => 1]);

foreach ($email->sent as $e) echo "  " . $e . PHP_EOL;
foreach ($inventory->changes as $c) echo "  " . $c . PHP_EOL;

// ── Strategy ─────────────────────────────────────────────────────────────────
echo PHP_EOL . "=== Strategy ===" . PHP_EOL;

interface SortStrategy {
    public function sort(array &$items): void;
    public function name(): string;
}

class PriceAscStrategy implements SortStrategy {
    public function sort(array &$items): void { usort($items, fn($a,$b) => $a["price"] <=> $b["price"]); }
    public function name(): string { return "Price ↑"; }
}

class NameStrategy implements SortStrategy {
    public function sort(array &$items): void { usort($items, fn($a,$b) => strcmp($a["name"], $b["name"])); }
    public function name(): string { return "Name A→Z"; }
}

class ValueStrategy implements SortStrategy {
    public function sort(array &$items): void {
        usort($items, fn($a,$b) => ($b["price"]*$b["stock"]) <=> ($a["price"]*$a["stock"]));
    }
    public function name(): string { return "Value ↓"; }
}

class ProductCatalogue {
    private SortStrategy $strategy;
    public function __construct(SortStrategy $strategy) { $this->strategy = $strategy; }
    public function setStrategy(SortStrategy $s): void { $this->strategy = $s; }
    public function sort(array $items): array {
        $this->strategy->sort($items);
        return $items;
    }
}

$products = [
    ["name" => "Surface Pro",  "price" => 864.00, "stock" => 15],
    ["name" => "Surface Pen",  "price" => 49.99,  "stock" => 80],
    ["name" => "Office 365",   "price" => 99.99,  "stock" => 999],
    ["name" => "USB-C Hub",    "price" => 29.99,  "stock" => 0],
];

$cat = new ProductCatalogue(new PriceAscStrategy());
foreach ([new PriceAscStrategy(), new NameStrategy(), new ValueStrategy()] as $strategy) {
    $cat->setStrategy($strategy);
    $sorted = $cat->sort($products);
    echo "  " . $strategy->name() . ": " . implode(", ", array_column($sorted, "name")) . PHP_EOL;
}

// ── Decorator ─────────────────────────────────────────────────────────────────
echo PHP_EOL . "=== Decorator ===" . PHP_EOL;

interface Logger {
    public function log(string $message): void;
    public function getLogs(): array;
}

class ConsoleLogger implements Logger {
    private array $logs = [];
    public function log(string $message): void {
        $this->logs[] = $message;
        echo "  [CONSOLE] " . $message . PHP_EOL;
    }
    public function getLogs(): array { return $this->logs; }
}

// Decorator: adds timestamp without modifying ConsoleLogger
class TimestampLogger implements Logger {
    public function __construct(private Logger $inner) {}
    public function log(string $message): void {
        $this->inner->log("[" . date("H:i:s") . "] " . $message);
    }
    public function getLogs(): array { return $this->inner->getLogs(); }
}

// Decorator: adds severity prefix
class LevelLogger implements Logger {
    public function __construct(private Logger $inner, private string $level = "INFO") {}
    public function log(string $message): void {
        $this->inner->log("[{$this->level}] " . $message);
    }
    public function getLogs(): array { return $this->inner->getLogs(); }
}

// Stack decorators — each wraps the previous
$logger = new LevelLogger(new TimestampLogger(new ConsoleLogger()), "INFO");
$logger->log("Order #1001 placed: Surface Pro ×2 = \$1728.00");
$logger->log("Payment processed via Stripe");

echo "  Total log entries: " . count($logger->getLogs()) . PHP_EOL;
'

📸 Verified Output:

=== Observer (Event System) ===
  ⚡ Webhook: refund for #1002
  Email: [order.placed] Order #1001 - Surface Pro
  Email: [order.shipped] Order #1001 - Surface Pro
  Email: [order.cancelled] Order #1002 - USB-C Hub
  Deduct stock: Surface Pro × 2

=== Strategy ===
  Price ↑: USB-C Hub, Surface Pen, Office 365, Surface Pro
  Name A→Z: Office 365, Surface Pen, Surface Pro, USB-C Hub
  Value ↓: Office 365, Surface Pro, Surface Pen, USB-C Hub

=== Decorator ===
  [CONSOLE] [INFO] [15:00:00] Order #1001 placed: Surface Pro ×2 = $1728.00
  [CONSOLE] [INFO] [15:00:00] Payment processed via Stripe

---

Summary

Pattern	Intent	PHP Use
Singleton	One instance globally	DB connections, config
Factory	Create objects by type	Gateways, drivers
Observer	Notify listeners of events	Order events, webhooks
Strategy	Swap algorithms at runtime	Sorting, pricing, export
Decorator	Add behaviour by wrapping	Logging, caching, auth
Command	Encapsulate an action	Queue jobs, undo/redo

Further Reading

• Refactoring.Guru PHP Patterns