An interface is a contract that defines the methods that a class must implement. A class can implement multiple interfaces; an abstract class is a semi-finished class that cannot be instantiated and can contain abstract methods and concrete implementations. Subclasses can only inherit one abstract class. For example, the Logger interface specifies a log method, and FileLogger implements it; Animal abstract class has abstract method makeSound and concrete method sleep, and Dog inherits and implements makeSound. Use interfaces to define common behaviors, such as payment interfaces; use abstract classes to suit shared logic, such as public methods of animal systems. Other details: The interface method defaults to public; abstract classes can have constructors; PHP8 supports interface default methods.

In PHP, interfaces and abstract classes are tools used to implement the "abstract" feature in object-oriented programming. They all help us define the structure of a class, but the usage scenarios and rules vary.

What are interfaces?

An interface is like a "contract", which specifies which methods the class must implement, but does not care about how these methods are implemented specifically. You can understand it as a norm.

• There can only be method declarations in the interface, not method bodies (before PHP 8)
• Classes implements interfaces
• A class can implement multiple interfaces

For example: If you write a Logger interface with log() method, then any class that implements this interface must provide a log() method.

 interface Logger {
    public function log($message);
}

class FileLogger implements Logger {
    public function log($message) {
        // Write log to file}
}

Interfaces are very suitable for defining some common behaviors, such as payment interfaces, authentication mechanisms, etc., so that different classes can provide services to the outside world in a unified way.

What is the difference between Abstract Classes?

An abstract class is a "semi-finished" class that cannot be instantiated, but can only be inherited. It can contain both abstract methods (no implemented methods) or already implemented methods.

• Abstract class is defined using abstract class
• Subclasses inherit abstract classes through extends
• A subclass can only inherit one abstract class

Let's give a simple example:

 abstract class Animal {
    abstract public function makeSound();

    public function sleep() {
        echo "Zzz...\n";
    }
}

class Dog extends Animal {
    public function makeSound() {
        echo "Woof!\n";
    }
}

Abstract classes are suitable when you have some shared logic but want subclasses to complete specific parts. For example, database operations, basic function packaging, etc.

Interface vs Abstract class: When to use which one?

It depends on your specific needs:

• Using interface :

  • Multiple completely unrelated classes need to implement the same behavior
  • Don't want to limit the inheritance structure of a class (because a class can implement multiple interfaces)

• Use abstract classes :

  • Need to share code logic to avoid duplication
  • There is a common parent class relationship between subclasses
  • I want to keep some implementation details

For example: a payment system can use interfaces to define payment behavior (such as pay() ), while an animal system may be more suitable to use abstract classes to share the logic of eat() or sleep() .

Tips: Some details that are easy to ignore

• All methods in the interface are public by default, and cannot be private or protected
• There can be constructors in abstract classes for initialization
• PHP does not support multiple inheritance, but can achieve similar effects through multiple interfaces.
• Starting with PHP 8, you can use default to implement methods (called "default methods") in the interface.

Basically that's it. Interfaces and abstract classes are not either one or the other relationship, but choose the right tool according to actual needs.

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