Working on inheritance page

assets/templates/_table_of_contents.phtml

@@ -11,8 +11,9 @@
         <li><a href="<?= page_path('loops') ?>">Loops</a></li>
         <li><a href="<?= page_path('arrays') ?>">Arrays</a></li>
         <li><a href="<?= page_path('functions') ?>">Functions</a></li>
-        <li><a href="<?= page_path('classes') ?>">Classes</a></li>
-        <li><a href="<?= page_path('classes-visibility') ?>">Classes - Visibility</a></li>
+        <li><a href="<?= page_path('classes') ?>">Classes: Introduction</a></li>
+        <li><a href="<?= page_path('classes-inheritance') ?>">Classes: Inheritance</a></li>
+        <li><a href="<?= page_path('classes-visibility') ?>">Classes: Visibility</a></li>
     </ol>
     <a href="<?= page_path('credits') ?>">Credits</a>
 </div>

code/classes-inheritance.php

@@ -0,0 +1,62 @@
+<?php
+
+// In PHP, a class can extend another class, inheriting the parent class'
+// properties and methods. To make a class a child of another, use the "extends"
+// keyword after the class name.
+class Vehicle
+{
+    public function drive()
+    {
+        echo "driving...\n";
+    }
+}
+
+class Truck extends Vehicle {}
+
+// This does not error because "Truck" extends "Vehicle".
+$truck = new Truck();
+$truck->drive();
+
+// Even though the child class inherits a parent class' properties and methods,
+// the child can still override the parent.
+class Tractor extends Vehicle
+{
+    public function drive()
+    {
+        echo "driving slowly...\n";
+    }
+}
+
+// Outputs "driving slowly..." instead of "driving...".
+$tractor = new Tractor();
+$tractor->drive();
+
+// A class can use a parent's property or method from the "$this" variable.
+class Motorcycle extends Vehicle
+{
+    public function pushPedal()
+    {
+        $this->drive();
+    }
+}
+
+// Outputs "driving...".
+$cycle = new Motorcycle();
+$cycle->pushPedal();
+
+// If you override a parent's property or method, the "$this" variable will refer to the child's
+// implementation of the property or method. To call the parent's property or method explicity,
+// use the "parent" keyword.
+class Racecar extends Vehicle
+{
+    public function drive()
+    {
+        parent::drive();
+
+        echo "driving even faster...\n";
+    }
+}
+
+// Outputs "driving..." and "driving even faster...".
+$racecar = new Racecar();
+$racecar->drive();

code/classes-visibility.php

@@ -33,11 +33,28 @@ class Phone2
     }
 }
 
-// The "protected" and "private" keywords work a little differently, but we
-// will learn more about "protected" when we discuss inheritance.
-// However, they both prevent functions and properties from being accessed outside an object.
+// The "protected" and "private" keywords work a little differently.
+// They both prevent functions and properties from being accessed outside an object.
+// However, a method or property marked "protected" can still be accessed by a child object.
 class Phone3
 {
     private $number;
+
     protected $caller;
+
+    public function setNumber($number)
+    {
+        $this->number = $number;
+    }
+}
+
+// In class "Smartphone", the "caller" property is accessible because the parent class
+// has it marked as "protected". However, "Smartphone" cannot access the the "number" property
+// because it is still listed as private.
+class Smartphone extends Phone3
+{
+    public function setCaller($caller)
+    {
+        $this->caller = $caller;
+    }
 }

config.php

@@ -79,13 +79,20 @@ return [
             'subtitle' => 'Object-oriented programming',
             'description' => '',
             'previous' => 'functions',
+            'next' => 'classes-inheritance',
+        ]),
+        Page::create('classes-inheritance', null, 'classes-inheritance.php', [
+            'title' => 'Classes Inheritance',
+            'subtitle' => 'Extend your objects',
+            'description' => '',
+            'previous' => 'classes',
             'next' => 'classes-visibility',
         ]),
         Page::create('classes-visibility', null, 'classes-visibility.php', [
             'title' => 'Classes Visibility',
             'subtitle' => 'Privatizing your objects',
             'description' => '',
-            'previous' => 'classes',
+            'previous' => 'classes-inheritance',
             'next' => '',
         ]),
     ],