Abstract Classes vs Interfaces in PHP

This question is frequently asked to test your understanding of object-oriented design and code structure.

🧩 Abstract Class — What is it?

An abstract class:

• Cannot be instantiated directly
• Can have defined methods and abstract methods (methods with no body)
• Can have properties
• Can contain constructor
• Supports inheritance

abstract class Animal {
    protected $name;

    public function __construct($name) {
        $this->name = $name;
    }

    abstract public function makeSound();

    public function getName() {
        return $this->name;
    }
}

class Dog extends Animal {
    public function makeSound() {
        return "Bark";
    }
}

⚙️ Interface — What is it?

An interface:

• Only contains method declarations, no implementation
• No properties (until PHP 8.1: readonly properties)
• A class can implement multiple interfaces

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

class FileLogger implements Logger {
    public function log($message) {
        echo "Writing to file: $message";
    }
}

🆚 Abstract Class vs Interface — Key Differences

Interview Tip:

If you need shared code with structure — use abstract classes.
If you need to define a contract for capabilities — use interfaces.