phpapprentice/code/interfaces.php

<?php

// The word "interface" is a confusing term because it is used for so many different concepts.
// Most often, we use it to describe the appearance of an app and how a user interacts with it.
// However, in PHP, an interface is a special construct that acts as a contract for classes.
// An interface defines what methods a class should have.
interface Chair
{
    public function setColor($color);
    public function setLegs($number);
}

// To use an interface with a class, you use the "implements" keyword after the class name.
// Now, the Recliner class must have a setColor method and a setLegs method.
// If you do not create the required methods on the class, PHP will throw an error.
class Recliner implements Chair
{
    private $color;
    private $legs;

    public function setColor($color)
    {
        $this->color = $color;
    }

    public function setLegs($number)
    {
        $this->legs = $number;
    }
}

// Interfaces are helpful when you are using code created by someone else.
// For example, another developer may have created code that manages online payments, but they want to give you
// the ability to create your own payment class that works with their code. In that case, the developer
// creates an interface with all the required methods they need to charge a payment. The interface
// becomes a contract between your code and the other developer's code to work a certain way.
interface Payment
{
    public function charge($amount);
}

class CreditCard
{
    public function charge($amount)
    {

    }
}

// Since CreditCard implements Payment, other developers can use the charge method, knowing it exists on the class.
$creditCard = new CreditCard();
$creditCard->charge(25);