Numbered basics chapters for sortability

chapters/basics/11-classes.md

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+Classes allow you to define your own data types. All classes start with the
+class keyword followed by the name of the class and opening and closing curly braces.
+```php
+<?php
+
+class Car
+{
+}
+```
+
+To create an instance of a class, you use the `new` keyword in front of the class name
+with parentheses.
+```php
+$car = new Car();
+```
+
+A class can define attributes and methods. An attribute is a piece of data
+stored on the class instance. You can define an attribute by adding the
+word `public` and a variable name inside the class definition.
+```php
+class Bicycle
+{
+    public $color;
+}
+```
+
+Then, when you create an instance of the class, you can set and use
+the attribute on the instance using `->`.
+```php
+$bike = new Bicycle();
+$bike->color = 'Blue';
+echo $bike->color . "\n";
+```
+
+An instance of a class is called an object. Congratulations!
+You are now performing object-oriented development.
+```php
+$redBike = new Bicycle();
+$redBike->color = 'Red';
+echo $redBike->color . " Bike Object\n";
+```
+
+A method is a function attached to the class. You can add a method
+to a class by using the `public` keyword followed by the function. A method
+can access the attributes and methods of an object instance using the `$this` variable.
+```php
+class Tricycle
+{
+    public $color;
+
+    public function echoColor()
+    {
+        echo $this->color . "\n";
+    }
+}
+```
+
+You can execute a method on an object using the same `->` arrow characters with parentheses after the method name.
+```php
+$bike = new Tricycle();
+$bike->color = 'Red';
+$bike->echoColor();
+```