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How do I apply SOLID principles in Laravel? Start with the change you expect to make, then shape responsibilities, contracts, and dependencies so that change stays local. SOLID is a set of design heuristics—not a requirement to create an interface, repository, or service for every class. In Laravel 13.x, use the service container for genuine dependency boundaries, keep controllers and application services focused, and verify behavior with the smallest test boundary that gives you confidence.
What SOLID means in PHP
SOLID names five object-oriented design principles commonly attributed to Robert C. Martin:
| Letter | Principle | Laravel question |
|---|---|---|
| S | Single Responsibility | Does this class have one coherent reason to change? |
| O | Open/Closed | Can a credible new variation be added without editing stable policy code? |
| L | Liskov Substitution | Can each implementation honor the contract’s behavioral expectations? |
| I | Interface Segregation | Does each client depend only on operations it uses? |
| D | Dependency Inversion | Does application policy depend on a useful abstraction rather than a vendor detail? |
These principles help you reason about change locality, coupling, substitution behavior, interface size, test boundaries, and the complexity an abstraction introduces. They do not promise faster software, stronger security, or a measurable business result; the cited guidance provides design definitions and framework instructions, not outcome statistics.
1. Single Responsibility Principle (SRP)
SRP says a class should have one responsibility, often expressed as one part of a specification or one reason for change. It does not mean one method per class. A responsibility is a coherent concern.
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A Laravel-shaped boundary
A controller can translate HTTP input into an application call and translate the result into an HTTP response. An application service can coordinate a use case. An adapter can handle a remote API’s protocol. If a payment-vendor API changes, you should not have to edit validation, order rules, and response formatting in the same class.
<?php
final class ConfirmOrder
{
public function __construct(private Notifier $notifier) {}
public function handle(Order $order): void
{
// Confirm the order, then delegate the notification boundary.
$this->notifier->send($order);
}
}
interface Notifier
{
public function send(Order $order): void;
}This is a teaching example, not a required Laravel structure; it was not executed or independently tested. Do not split a small, stable class merely to satisfy a slogan. Ask which specification area would force a change.
2. Open/Closed Principle (OCP)
Software entities should be open for extension and closed for modification. In practice, this is useful when a real variation is likely: for example, adding a second payment provider or notification channel.
Keep provider choices at a composition boundary
Let the use case depend on a stable contract and select the provider in configuration or a service provider. A new implementation can then be added without scattering provider-specific conditionals through business policy. OCP is not a mandate for a plugin architecture when there is only one implementation and no credible variation.
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3. Liskov Substitution Principle (LSP)
LSP means an object should be replaceable by an instance of its subtype without changing program correctness. PHP signatures are only the starting point. Implementations must agree on valid inputs, returned values, side effects, and error behavior.
A behavioral contract
If Notifier::send() promises to accept every valid order and either complete or report a defined failure, an implementation that silently drops some orders, returns a different type, or throws an undocumented exception surprises its caller. Such a class may satisfy the interface syntactically while violating LSP.
- Document preconditions callers can rely on.
- Keep return and exception behavior compatible.
- Test each implementation against the same contract-level expectations.
4. Interface Segregation Principle (ISP)
Prefer client-specific interfaces to one general-purpose interface. A reporting client that only reads invoices should not depend on unrelated write or deletion operations because a broad interface grouped them together.
Split by consumer needs
Small interfaces make substitutes easier to write and reduce accidental coupling. Split an interface when clients genuinely have different capabilities or change for different reasons. Do not create dozens of interfaces whose only purpose is to wrap one private class; indirection without a client need adds cost.
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5. Dependency Inversion Principle (DIP)
DIP says high-level policy should depend on abstractions, while implementation details depend on those abstractions. Laravel’s service container supplies dependency injection: dependencies can be provided through constructors (and, in some cases, setters). Injection is the mechanism; inversion is the design choice about what the policy is coupled to.
Constructor injection and bindings
Laravel can often auto-resolve classes with no dependencies or only concrete dependencies. Controllers, middleware, event listeners, and queued-job handlers may type-hint those dependencies. When a class type-hints an interface, register the interface-to-implementation mapping, typically in a service provider.
use IlluminateSupportServiceProvider;
final class AppServiceProvider extends ServiceProvider
{
public function register(): void
{
$this->app->bind(Notifier::class, EmailNotifier::class);
}
public function boot(): void
{
// Routes and event listeners do not belong in register().
}
}Laravel’s service-provider guidance reserves register for container bindings. If a concrete dependency is stable and there is no meaningful substitution boundary, injecting that concrete class can be the simpler and more honest design. An interface is not automatically an improvement.
Dependency injection is not dependency inversion
Consider a use case that receives a concrete StripeNotifier through its constructor. It uses injection, but its policy still knows a vendor detail. Replacing that detail with a small Notifier abstraction can invert the dependency when the application genuinely needs another channel, an isolation seam, or a separate adapter. Conversely, adding an interface solely to make the diagram look inverted creates configuration and indirection without reducing a current or credible change.
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Testing SOLID designs in Laravel
Laravel supports PHPUnit and Pest and documents php artisan test. Unit tests isolate small behavior, often one method. Feature tests exercise broader object interaction or a complete HTTP request. Laravel’s testing guide says most tests should generally be feature tests because they provide the most confidence in overall behavior; that is guidance, not a rule that eliminates unit tests.
A practical test split
- Use a substitute notifier to unit-test the order-confirmation policy when notification is a real boundary.
- Add a feature test for the route, validation, persistence, and visible response.
- Run the suite with
php artisan testafter changing a binding or implementation.
Mocking is valuable when it isolates a remote or slow boundary. Do not mock every collaborator by default; a feature test often catches wiring mistakes that an isolated unit test cannot.
How to choose the simplest design for a change request
- State the change. “Add SMS notifications” is more actionable than “make this SOLID.”
- Find reasons for change. Separate HTTP translation, business policy, and vendor protocol only when they evolve independently.
- Look for real variation. A second provider, deployment-specific implementation, or test substitute can justify a contract.
- Check substitutability. Write down valid inputs, outputs, side effects, and failures before adding implementations.
- Minimize interfaces. Expose only operations each client needs.
- Choose the test boundary. Prefer a feature test for end-to-end confidence and a unit test for focused policy or expensive external behavior.
- Count the indirection. If the abstraction does not localize a current or credible change, keep the concrete class.
Common Laravel mistakes
- Applying five rules mechanically: SOLID is a way to notice design pressure, not a compliance scorecard.
- Making every class “single method”: Several methods can form one cohesive responsibility.
- Confusing inheritance with LSP: Matching method signatures does not prove behavioral compatibility.
- Creating repository and service layers by reflex: They add value only when they isolate a meaningful boundary or change.
- Assuming the container designs the system: It resolves dependencies; developers still choose responsibilities and contracts.
- Registering application work in a provider’s
registermethod: Keep that method for bindings, not routes or listeners.
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Does every Laravel interface need a service-provider binding?
No. Laravel often auto-resolves concrete classes. Bind an interface when the container needs to know which implementation to use.
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Should I use unit tests or feature tests first?
Choose the boundary that proves the behavior: focused unit tests for isolated policy and feature tests for wiring, interactions, and HTTP behavior. Laravel generally recommends feature tests for broad confidence.
Is a repository required to follow SOLID?
No. Add a repository only when it isolates a meaningful persistence boundary or change; SOLID does not prescribe named layers.
The Bottom Line
Apply SOLID by tracing reasons for change: keep responsibilities cohesive, introduce contracts only at real substitution boundaries, honor behavioral expectations, and let Laravel’s container compose the result. The most maintainable Laravel code is usually the simplest design that keeps the next credible change local and testable.
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