PHP Course : Abstract Classes vs Interfaces

TLDR; With Abstract Classes, you can define default behaviours that can be inherited by subclasses. That's not possible with interfaces. The template design pattent makes use of abstract classes. It defines default behaviour in the parent class but also abstract method that are meant to be implemented in the sub classes. You cannot do that with Interfaces, which are more suited to defining types.

Abstract Classes

An Abstract class provides a template for any sub classes.

Let's make two different Car class :

class Audi {
  public function description() {
    return "German car";
  }
}

class Volvo {
  public function description() {
    return "Swedish car";
  }
}

Immediately you see the repetition. You must not repeat your code over and over if you want to add feature to a Car class. You need a parent class to bind them.

Let's make a simple parent class then:

class Car {
  public function brand() {
	return (new ReflectionClass($this))->getShortName();
  }
}

class Audi extends Car {
  public function description() {
    return "German car";
  }
}

class Volvo extends Car {
  public function description() {
    return "Swedish car";
  }
}

We can now do this:

$volvo = new Volvo;
$volvo->brand(); // prints "Volvo";

But this code also allows us to do this $car = new Car and we should not be able to do that. It doesn't really make sense.

The solution : the abstract class ! Let's switch the Car class to an Abstract Class :

abstract class Car {
  public function brand() {
	return (new ReflectionClass($this))->getShortName();
  }
}

class Audi extends Car {
  public function description() {
    return "German car";
  }
}

class Volvo extends Car {
  public function description() {
    return "Swedish car";
  }
}

Now you won't be able to instanciate the Car class anymore.

Defining methods

The purpose of the abstract is not only to be a Parent class, but also allows you to pre define methods. Sometimes with existing logic, sometimes not.

Let's update our Car abstract class

abstract class Car {
  public function brand() {
	return (new ReflectionClass($this))->getShortName();
  }
  
  public function logo() {
	// we can write some default logic here
	return strtolower($this->brand()) . '.png';
  }
  
  abstract public function description();
}

class Audi extends Car {
  public function logo() {
	// we overwrite this method from the abstract class
	return 'custom-audi-logo.svg';
  }
  
  public function description() {
    return "German car";
  }
}

class Volvo extends Car {
  public function description() {
    return "Swedish car";
  }
}

Interfaces

Interfaces simply does not let you write any default logic. The same example with Interface would be like this:

Interface Car {
  public function brand(): string;
  
  public function logo(): string;
  
  public function description(): string;
}

class Audi extends Car {
  public function brand() {
	return 'Audi';
  }
  
  public function logo() {
	return 'custom-audi-logo.svg';
  }
  
  public function description() {
    return "German car";
  }
}

For more information about interfaces you can read my course on Interface Segregation.