Interfaces define interchangeable public capabilities, abstract classes share real invariants and algorithms, traits reuse narrow implementation fragments, and composition delegates work to explicit collaborators.
Select each mechanism by ownership and substitution needs, label PHP 8.4 interface-property syntax, keep contracts small, and verify every implementation with shared behavior tests.
<?php
interface MessageSender
{
public function send(string $recipient, string $message): void;
}
final class WelcomeService
{
public function __construct(private MessageSender $sender)
{
}
public function welcome(string $email): void
{
$this->sender->send($email, 'Welcome');
}
}WelcomeService depends on the capability, not a specific email provider.
An abstract class cannot be instantiated. It can require abstract methods while providing final workflow steps or protected extension hooks. Use it when implementations truly share a lifecycle and state.
<?php
abstract class Report
{
final public function render(): string
{
return strtoupper($this->title()) . PHP_EOL . $this->body();
}
abstract protected function title(): string;
abstract protected function body(): string;
}A trait shares method implementation without creating an is-a relationship. Keep traits cohesive and avoid using them as hidden containers for unrelated dependencies.
<?php
trait FormatsTimestamp
{
private function formatTime(DateTimeImmutable $time): string
{
return $time->format(DATE_ATOM);
}
}| Need | Choose |
|---|---|
| Several unrelated classes expose one capability | Interface |
| A family shares state and workflow | Abstract class |
| Classes need one small method implementation | Trait |
| One object needs another service | Composition with an interface |
A class may implement several interfaces but extend only one class. Keep interfaces focused on caller needs rather than mirroring every public method of one implementation. Return and parameter types make the promise useful; vague mixed signatures push uncertainty to runtime.
An abstract template method can control a stable algorithm while subclasses provide one step, but deep inheritance makes behavior difficult to trace. Prefer composition when variants can be supplied as collaborating objects or when implementations need to combine behaviors independently.
An interface defines public behavior that unrelated classes can implement. Consumers type against the interface and do not need to know storage, network, or vendor details. A useful interface represents one cohesive capability rather than every method an implementation happens to expose.
Implementations must provide compatible method signatures. Parameter and return variance follow PHP compatibility rules, and visibility must satisfy the public contract. Because named arguments let callers depend on parameter names, keep implementation parameter names aligned with the interface.
A class may implement several interfaces, and an interface may extend one or more interfaces. Resolve same-name methods only when one implementation can satisfy all inherited signatures. Conflicting contracts are a design problem, not something to hide with loose types.
Interface constants are available to implementers and callers, but configuration and domain values often belong in enums or dedicated value objects. Do not use an interface as a miscellaneous constant container.
An abstract class can hold state, constructor logic, concrete methods, protected extension points, and abstract methods. It cannot be instantiated directly. Use it when implementations share a real invariant and algorithm, not merely because two classes contain similar lines.
A template method can make a workflow final or stable while delegating selected steps to protected abstract methods. Document which steps may vary, what each returns, whether exceptions are allowed, and which state is valid before and after the hook.
Abstract state creates construction coupling. Subclasses must honor parent initialization and cannot extend another base class. Keep required dependencies explicit, prefer private parent state with protected operations over exposed mutable fields, and avoid calling overridable methods from constructors.
An abstract class may implement only part of an interface, leaving the remaining methods to concrete subclasses. This can be useful for a verified common implementation, but the final class still needs contract tests for every public method.
A trait inserts methods and properties into a class for horizontal reuse. It is not a runtime type and cannot be passed as a dependency contract. Use traits for small implementation mechanisms whose required collaborators and state remain obvious.
When two traits provide the same method, resolve the conflict with `insteadof`; use `as` to add an alias or adjust visibility under allowed rules. The resolution should explain the chosen behavior. A dense conflict block is a signal that composition through objects may be clearer.
Traits can declare abstract requirements and access class members, which creates an implicit contract. Keep those requirements narrow and documented. If a trait expects many specially named properties and methods, extract a service with an explicit interface and constructor dependency.
Trait static properties and method behavior can interact with inheritance according to the running PHP version. Avoid shared mutable trait state for request or tenant data. Prefer instance collaborators whose lifetime and tests are explicit.
From PHP 8.4, an interface may declare a public property contract that is readable, writable, or both through property-hook syntax. The contract describes public access, not one required storage technique. A traditional public property, compatible hooked property, virtual property, or readonly property in a read-only contract can satisfy it.
A settable interface property cannot be satisfied by a readonly property because callers are promised public write access. Keep property types and hook behavior compatible, and do not hide expensive I/O behind property syntax when a method would communicate cost and failure more clearly.
Property contracts can be useful for simple value-oriented APIs, but methods often express validation, failure, and capability better. Do not migrate every getter and setter merely because the syntax exists. State the minimum supported PHP version before using interface properties.
Libraries supporting PHP versions before 8.4 must use method contracts or separate version branches. Syntax cannot be conditionally parsed by an older runtime. Build and test with the actual minimum and current runtime, not only the developer machine.
Choose an interface when clients need interchangeable behavior, an abstract class when subclasses share a protected invariant and algorithm, a trait for a small reusable implementation fragment, and composition when a class should delegate to independently testable collaborators. These tools can be combined but should not overlap without purpose.
Depend on the narrowest contract used by the consumer. An application service that only writes audit events should not receive an administrative repository interface with deletion and migration methods. Smaller contracts reduce accidental coupling and make fakes straightforward.
Do not create one interface for every concrete class automatically. A contract earns its place when it protects a boundary, supports multiple implementations, stabilizes tests, or expresses a capability consumed independently. Premature interfaces can duplicate every change without improving design.
Use final classes by default where extension is not an intended feature. Inheritance is a public customization surface with compatibility obligations. Composition usually offers safer substitution without exposing protected internals.
Write shared contract tests that run against each implementation. Verify normal results, invalid input, failure translation, side effects, ordering, idempotency, and cleanup promised by the interface. A fake that passes tests while violating production semantics weakens the contract.
For abstract templates, test a minimal concrete subclass and each real subclass. Confirm hooks are called in the intended order and cannot leave partial state. For traits, create a tiny host class that exposes the behavior under test without relying on an unrelated production class.
Static analysis catches incompatible signatures, missing methods, impossible property contracts, and variance errors. Mutation tests or adversarial fakes can reveal tests that assert only types and never behavior. Keep examples valid on the minimum supported PHP version.
Refactor safely by adding the new contract, adapting one implementation, moving consumers, and deleting the old surface only after repository-wide search and tests show no use. Avoid one large inheritance rewrite that changes behavior and architecture simultaneously.
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