Interfaces

Interfaces are similar to abstract classes, but they are not classes. They are a way to define a contract for a class to follow without having to define the implementation. An interface is a collection of abstract methods and constants. A class that implements an interface must implement all the methods and constants defined in the interface. An interface cannot be instantiated. It can only be implemented by a class.

Example

Let's look at an example. Imagine you're building a content management system (CMS) where you have various types of content, such as articles, videos, and images. Each type of content should have the ability to be displayed and edited. You want to ensure that all content types adhere to a common set of methods for displaying and editing content. You can use an interface to define the contract for displaying and editing content.

Define Interface

We use the interface keyword to define an interface.

interface ContentInterface {
    public function display();
    public function edit();
}

Implement Interface

Let's create our Article class that implements the ContentInterface interface. We use the implements keyword to do so:

class Article implements ContentInterface {
    private $title;
    private $content;

    public function __construct($title, $content) {
        $this->title = $title;
        $this->content = $content;
    }

    public function display() {
        echo "<h2>{$this->title}</h2>";
        echo "<p>{$this->content}</p>";
    }

    public function edit() {
        echo "Editing the article '{$this->title}'...";
    }
}

Let's create another class called Video that implements the ContentInterface interface:

class Video implements ContentInterface {
    private $title;
    private $url;

    public function __construct($title, $url) {
        $this->title = $title;
        $this->url = $url;
    }

    public function display() {
        echo "<h2>{$this->title}</h2>";
        echo "<iframe src='{$this->url}'></iframe>";
    }

    public function edit() {
        echo "Editing the video '{$this->title}'...";
    }
}

The video class has a display() method that displays the video using an iframe.

Instantiate Objects

Let's instantiate an Article object and a Video object:

$article = new Article('Introduction to PHP', 'PHP is a versatile scripting language...');
$video = new Video('PHP For Beginners', 'https://www.youtube.com/watch?v=BUCiSSyIGGU');

Let's add the display() functions to the template to output:

 <div class="container mx-auto p-4 mt-4">
    <div class="bg-white rounded-lg shadow-md p-6 mt-6">
      <?= $article->display(); ?>
    </div>
    <div class="bg-white rounded-lg shadow-md p-6 mt-6">
      <?= $video->display(); ?>
    </div>
  </div>

If you want, you can call the edit() method like this:

$article->edit();
$video->edit();

So this is an example of how we can use an interface to define a contract for a class to follow. The Article and Video classes both implement the ContentInterface interface, which means they must implement the display() and edit() methods. This ensures that all content types have the ability to be displayed and edited.