When to implement and extend? [closed]

Answer 1

Inheritance is useful to reduce the amount of code you rewrite. If you have several classes with a few common methods or fields, instead of defining these methods and fields over and over you can factor them into a base class and have each of the child classes extend that base class.

Interfaces (and implements) are useful when you'd like to define a common protocol for how a group of objects should behave. For example, you might want to mandate that objects that are comparable can be compared for equality and hashed, etc.

Using inheritance is ultimately a design choice. Be on the lookout for cases where you define the same methods across several classes; those are excellent cases where you can factor those methods out into a base class. The same goes for classes that observe some of the same characteristics: you can guarantee consistency by putting those characteristics in an interface to be implemented by those related classes.

Inheritance is a big concept in OOP that goes way beyond just PHP. I recommend you read the wikipedia article on inheritance and perhaps Design Patterns by the Gang of Four.

I believe your understanding of inheritance is mainly correct. The next step would be to use it in production.

Answer 2

Implement:

Interfaces are abstract classes, so you can only declare things. A class implements an interface. You can implement multiple interfaces.

Extend:

You extend classes when you want a more specific version of a class.

Example:

// Contract: a pet should play
public interface Pet {
    public void play(); 
}

// An animal eats and sleeps
class Animal {
    public void eat();
    public void sleep();
}


public class Camel extends Animal {

}

public class Dog extends Animal implements Pet {    
    public void play() {...}
}

Both Camel and Dog are animals, so they extend Animal class. But only the Dog is a specific kind of Animal that also can be a Pet