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content/16-interfaces.md

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+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.
+```php
+<?php
+
+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.
+```php
+class Recliner implements Chair
+{
+    private $color;
+    private $legs;
+
+    public function setColor($color)
+    {
+        $this->color = $color;
+    }
+
+    public function setLegs($number)
+    {
+        $this->legs = $number;
+    }
+}
+```
+
+Interfaces are helpful when you are using code created by someone else.
+For example, another developer may have created code that manages online payments, but they want to give you
+the ability to create your own payment class that works with their code. In that case, the developer
+creates an interface with all the required methods they need to charge a payment. The interface
+becomes a contract between your code and the other developer's code to work a certain way.
+```php
+interface Payment
+{
+    public function charge($amount);
+}
+
+class CreditCard implements Payment
+{
+    public function charge($amount)
+    {
+        // contacts a credit card payment provider...
+    }
+}
+```
+
+Since `CreditCard` implements `Payment`, a developer can check that it implements `Payment` and then use the `charge` method knowing the function exists on the class.
+```php
+$creditCard = new CreditCard();
+if ($creditCard instanceof Payment) {
+    $creditCard->charge(25);
+}
+```