Interfaces and Dependency Inversion

<?php
// Abstraction (interface)
interface CacheInterface {
    public function get(string $key): mixed;
    public function set(string $key, mixed $value, int $ttl = 3600): void;
    public function forget(string $key): void;
}

// Two implementations
class RedisCache implements CacheInterface { /* ... */ }
class ArrayCache implements CacheInterface { /* ... */ } // for tests

// High-level module depends on interface, not concrete class
class ProductService {
    public function __construct(private CacheInterface $cache) {}

    public function getProduct(int $id): array {
        $key = "product:{$id}";
        $cached = $this->cache->get($key);
        if ($cached !== null) return $cached;
        // ... fetch from DB ...
        $this->cache->set($key, $product);
        return $product;
    }
}