PHP Design Patterns: Singleton, Factory, Observer, Strategy and More

Why Design Patterns?

Design patterns are reusable solutions to common problems. They give developers a shared vocabulary and prevent reinventing the wheel. Senior PHP developers are expected to know and apply them.

Singleton — One Instance Only

<?php
class Database {
    private static ?self $instance = null;
    private PDO $pdo;

    private function __construct() {
        $this->pdo = new PDO('mysql:host=localhost;dbname=app', 'user', 'pass');
    }
    private function __clone() {}

    public static function getInstance(): self {
        if (self::$instance === null) {
            self::$instance = new self();
        }
        return self::$instance;
    }

    public function query(string $sql, array $params = []): array {
        $stmt = $this->pdo->prepare($sql);
        $stmt->execute($params);
        return $stmt->fetchAll(PDO::FETCH_ASSOC);
    }
}

$db    = Database::getInstance();
$users = $db->query('SELECT * FROM users WHERE active = ?', [1]);

Factory — Create Without new

<?php
interface Notifier {
    public function send(string $to, string $message): bool;
}

class EmailNotifier implements Notifier {
    public function send(string $to, string $message): bool {
        echo "Email to $to: $message\n";
        return true;
    }
}

class SMSNotifier implements Notifier {
    public function send(string $to, string $message): bool {
        echo "SMS to $to: $message\n";
        return true;
    }
}

class NotifierFactory {
    public static function create(string $type): Notifier {
        return match($type) {
            'email' => new EmailNotifier(),
            'sms'   => new SMSNotifier(),
            default => throw new InvalidArgumentException("Unknown: $type"),
        };
    }
}

$n = NotifierFactory::create(config('notify.default'));
$n->send('rahul@example.com', 'Welcome to EzyCoders!');

Observer — Event-Driven

<?php
interface Observer {
    public function update(string $event, mixed $data): void;
}

class EventBus {
    private array $listeners = [];

    public function on(string $event, Observer $obs): void {
        $this->listeners[$event][] = $obs;
    }

    public function emit(string $event, mixed $data = null): void {
        foreach ($this->listeners[$event] ?? [] as $obs) {
            $obs->update($event, $data);
        }
    }
}

class WelcomeEmailer implements Observer {
    public function update(string $event, mixed $data): void {
        echo "Sending welcome email to {$data['email']}\n";
    }
}

class ReputationGranter implements Observer {
    public function update(string $event, mixed $data): void {
        echo "Granting 50 points to user {$data['id']}\n";
    }
}

$bus = new EventBus();
$bus->on('user.registered', new WelcomeEmailer());
$bus->on('user.registered', new ReputationGranter());
$bus->emit('user.registered', ['id'=>1,'email'=>'r@example.com']);

Repository — Abstract Data Access

<?php
interface PostRepository {
    public function findById(int $id): ?array;
    public function findPublished(): array;
    public function save(array $post): int;
}

class MysqlPostRepository implements PostRepository {
    public function __construct(private PDO $db) {}
    
    public function findById(int $id): ?array {
        $s = $this->db->prepare('SELECT * FROM posts WHERE id=?');
        $s->execute([$id]);
        return $s->fetch(PDO::FETCH_ASSOC) ?: null;
    }
    
    public function findPublished(): array {
        return $this->db->query("SELECT * FROM posts WHERE status='published'")->fetchAll(PDO::FETCH_ASSOC);
    }
    
    public function save(array $post): int {
        $s = $this->db->prepare('INSERT INTO posts (title,content) VALUES (?,?)');
        $s->execute([$post['title'], $post['content']]);
        return (int) $this->db->lastInsertId();
    }
}