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content/13-classes-visibility.md

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+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.
+```php
+<?php
+
+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.
+```php
+$phone = new Phone();
+$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.
+```php
+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.
+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 class.
+```php
+class Phone3
+{
+    private $number;
+
+    protected $caller;
+
+    public function setNumber($number)
+    {
+        $this->number = $number;
+    }
+}
+```
+
+In class `Smartphone`, the `caller` property is accessible because the parent class
+has it marked as `protected`. However, `Smartphone` cannot access the `number` property
+because it is still listed as private.
+```php
+class Smartphone extends Phone3
+{
+    public function setCaller($caller)
+    {
+        $this->caller = $caller;
+    }
+}
+```