Added page for interfaces

assets/templates/_table_of_contents.phtml

@@ -20,6 +20,7 @@
         <li><a href="<?= page_path('classes-visibility') ?>">Classes: Visibility</a></li>
         <li><a href="<?= page_path('classes-constructor') ?>">Classes: Constructor</a></li>
         <li><a href="<?= page_path('static') ?>">Static</a></li>
+        <li><a href="<?= page_path('interfaces') ?>">Interfaces</a></li>
     </ol>
     <a href="<?= page_path('credits') ?>">Credits</a>
 </div>

code/interfaces.php

@@ -0,0 +1,52 @@
+<?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);

config.php

@@ -135,6 +135,12 @@ return [
             'title' => 'Static',
             'subtitle' => 'Class properties and methods',
             'previous' => 'classes-constructor',
+            'next' => 'interfaces',
+        ]),
+        Page::create('interfaces', null, 'interfaces.php', [
+            'title' => 'Interfaces',
+            'subtitle' => 'Writing code contracts',
+            'previous' => 'static',
             'next' => '',
         ]),
     ],