Wrote inital content for classes-visibility and tweaked classes page

code/classes-visibility.php

@@ -0,0 +1,43 @@
+<?php
+
+// In the last chapter, we defined properties and methods on the class using the public keyword.
+// You can also define them using the "protected" and "private" keywords.
+// Both keywords prevent the properties and functions from being accessible outside the object.
+// Only the object itself can use each.
+class Phone
+{
+    private $number;
+
+    public function setNumber($number)
+    {
+        $this->number = $number;
+    }
+}
+
+// We cannot set the number using "$phone->number = '123-456-7890'".
+// Instead, we can use the public method.
+$phone = new Phone();
+$phone->setNumber('123-456-7890');
+
+// Making an attribute or function private, gives you more control over the data in the object.
+// For example, we could prevent a number being set if it starts with a 7.
+class Phone2
+{
+    private $number;
+
+    public function setNumber($number)
+    {
+        if (substr($number, 0, 1) !== '7') {
+            $this->number = $number;
+        }
+    }
+}
+
+// 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.
+class Phone3
+{
+    private $number;
+    protected $caller;
+}

code/classes.php

@@ -24,9 +24,15 @@ $bike = new Bicycle();
 $bike->color = 'Blue';
 echo $bike->color . "\n";
 
+// An instance of a class is called an object. Congratulations!
+// You are now performing object-oriented development.
+$redBike = new Bicycle();
+$redBike->color = 'Red';
+echo $redBike->color . " Bike Object\n";
+
 // A method is a function attached to the class. You can add a method
 // to a class by using the "public" keyword followed by the function. A method
-// can access the attributes and methods of the instance using the "$this" variable.
+// can access the attributes and methods of an object instance using the "$this" variable.
 class Tricycle
 {
     public $color;