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How do abstract classes differ from interfaces in PHP, and when would you use each?
How do abstract classes differ from interfaces in PHP, and when would you use each?
Jun 04, 2025 pm 04:37 PM
Abstract classes and interfaces have their own uses in PHP. 1. Abstract classes are used to share code, support constructors, and control access, and contain abstract methods and concrete methods. 2. The interface is used to define behavior contracts. All methods must be implemented and are public by default, and support multiple inheritance. 3. Since PHP 8, the interface can contain default methods to implement, but there is still no constructor or state. 4. When using abstract classes, you need to encapsulate implementation details; when using interfaces, you need to define cross-class behavior or build plug-in systems. 5. Can be used in combination: abstract classes implement interfaces or combine multiple interfaces into one abstract class. Select whether the structure plus sharing behavior (abstract class) or only the structure (interface).
Abstract classes and interfaces in PHP both help enforce structure in object-oriented programming, but they serve different purposes and are used in different scenarios. Let's break it down.
What Abstract Classes Can Do
An abstract class is a class that cannot be instantiated on its own and must be extended by other classes. It can contain both implemented methods and abstract methods (methods without a body).
- You can have method implementations – This means you can write actual code inside an abstract class.
- It supports constructors – So you can initialize values ??when a child class is created.
- You can define protected or private methods – Which gives you more control over visibility.
For example:
abstract class Animal {
abstract public function makeSound();
public function sleep() {
echo "Zzz...";
}
} Here, makeSound() must be implemented by subclasses, but sleep() already has a body.
Use abstract classes when:
- You want to share code among closely related classes.
- You need to enforce a template but allow for some default behavior.
- You want to control access with protected/private methods.
What Interfaces Are For
An interface defines a contract — any class that implements the interface must provide the methods defined in it. But unlike abstract classes, interfaces can't have any implementation at all (prior to PHP 8.0; more on that later).
- Only method signatures – No implementation details allowed.
- Support multiple inheritance – A class can implement multiple interfaces.
- All methods must be public – No protected or private methods.
Example:
interface Logger {
public function log($message);
}A class using this would look like:
class FileLogger implements Logger {
public function log($message) {
// Write to file
}
}Interfaces are best when:
- You're defining behaviors that unrelated classes might need.
- You want to ensure certain methods exist across different parts of your app.
- You're designing plugins or APIs where implementation details don't matter, only method names do.
PHP 8 Changes: Default Methods in Interfaces
Starting from PHP 8, interfaces can now include method bodies. That means interfaces can offer default implementations , which blurs the line between interfaces and abstract classes a bit.
interface Renderer {
public function render();
public function getType() {
return 'default';
}
}This makes interfaces more flexible, but there are still key differences:
- Interfaces still can't have constructors.
- They still can't have state (like properties), unless using traits or workarounds.
- You still can't restrict method visibility in interfaces.
So while interfaces gained some power, abstract classes are still better if you need internal logic and constructor support.
When to Use Each
In general:
Use abstract classes when:
- You have shared logic between similar objects.
- You need to encapsulate some implementation details.
- You want to define protected or private helper methods.
Use interfaces when:
- You want to define a behavior that many unrelated classes should follow.
- You're building something extensible, like a plugin system.
- You want to decouple your code from specific implementations.
Also, consider combining them:
- An abstract class can implement an interface.
- Or multiple interfaces can be grouped into one abstract class for convenience.
Basically, pick based on whether you need to enforce structure alone (interface) or structure plus some shared behavior (abstract class).
The above is the detailed content of How do abstract classes differ from interfaces in PHP, and when would you use each?. For more information, please follow other related articles on the PHP Chinese website!
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