phpapprentice/chapters/classes-visibility.md

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;
    }
}
```