Laravel's service container is the backbone of dependency injection. It resolves classes and their dependencies automatically, including all of Laravel's own components. Bind implementations with $this->app->bind(Interface::class, Implementation::class); use singleton() instead of bind() when the same instance should be returned on every resolution. Service providers are the central place to register bindings in register() and configure services in boot(), which runs after all providers register. Facades provide a static-like syntax (Cache::get('key')) on top of container-resolved classes, concise yet still testable. Real-time facades extend this pattern: prefix any class with Facades\ in a use statement and Laravel dynamically generates a facade that resolves the class from the container, letting you swap implementations in tests without code changes.
Validation guards incoming data. Inline rules run via $request->validate(['title' => 'required|max:255']); for reusable logic, generate a Form Request with make:request StorePostRequest, define rules in rules(), and type-hint it in the controller so Laravel resolves and validates it automatically. Common rules include required, string|max:255, email, unique:users,email, confirmed (matches a field_confirmation value), exists:posts,id, and nullable|image|mimes:jpg,png. Custom rules are first-class: make:rule Uppercase generates a class whose validate() method calls $fail(...) on invalid input; apply with 'name' => [new Uppercase]. For custom 404 and 500 pages, drop Blade files at resources/views/errors/{code}.blade.php; for JSON responses, throw abort(response()->json(...), 422) or hook withExceptions() in bootstrap/app.php.
Testing is integrated end-to-end. Feature tests generated with make:test UserTest use HTTP testing helpers like $this->get('/')->assertStatus(200). The RefreshDatabase trait wraps each test in a transaction that rolls back at the end, keeping tests isolated without re-running migrations. Authenticate routes with \(this->actingAs(\)user), or for Sanctum tokens: \(this->actingAs(\)user, 'sanctum'). Mock collaborators with \(this->mock(Service::class, fn (\)m) => $m->shouldReceive(...)); use Bus::fake(), Event::fake(), and Notification::fake() to assert framework interactions without executing them. For HTTP client tests, Http::fake([...]) and Http::assertSent(...) intercept requests; pair with Http::preventStrayRequests() to catch unwired external calls. Format code with Laravel Pint (vendor/bin/pint, with --test to report without fixing) or PHP_CodeSniffer for stricter PSR-12 enforcement.
Cross-cutting concerns cover caching, mail, storage, logging, and configuration. The Cache facade offers Cache::remember($key, $ttl, $cb) for memoized lookups, rememberForever() for indefinite storage, atomic Cache::lock('key', 10)->block(5, $cb) for race-free operations, and Cache::tags(['users'])->flush() for tag-based eviction on supported stores. Mail supports raw messages via Mail::raw or full Mailable classes generated with make:mail that implement envelope(), content(), and attachments(); add ShouldQueue to send asynchronously. Markdown mail renders pre-styled HTML; publish templates with vendor:publish --tag=laravel-mail to customize themes. Storage uses disks (local, public, s3): php artisan storage:link makes the public disk accessible at /storage. File uploads go through $request->file('avatar')->store('avatars', 'public') after validating with image|max:2048; helper methods include getClientOriginalName(), extension(), size(), and isValid(). Laravel's HTTP client (Http::get(...)) supports timeouts, retries, bearer tokens, and throw() for 4xx/5xx handling. Logging routes through configured channels (single, daily, slack, stack) via the Log facade, with contextual arrays such as Log::info('User signed in', ['id' => $u->id]). Always read config via config('app.timezone') and never env() outside config files, because php artisan config:cache causes env() calls elsewhere to return null in production.