Numbered basics chapters for sortability

chapters/basics/17-abstract.md

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+Abstract classes are similar to interfaces in that they define methods that a sub-class must implement.
+However, an abstract class can also have normal methods. To create an abstract class, use the `abstract`
+keyword followed by `class` and the name of the class.
+```php
+<?php
+
+abstract class CellPhone
+{
+    abstract public function unlock();
+
+    public function turnOn()
+    {
+        echo "Holding power button...\n";
+    }
+}
+```
+
+To use an abstract class, you create another class that extends it and create any methods that were marked as abstract.
+A class can only extend one abstract class and the child class has to implement all abstract methods.
+```php
+class iPhone extends CellPhone
+{
+    public function unlock()
+    {
+        echo "Touching fingerprint reader...\n";
+    }
+}
+```
+
+In this example, we use an abstract class to create the behavior for turning on a cell phone and then
+force the child classes to implement how to unlock the phone. We have clearly defined what a cell phone
+performs and we have limited code duplication.
+```php
+class Android extends CellPhone
+{
+    public function unlock()
+    {
+        echo "Typing passcode...\n";
+    }
+}
+```
+
+Our iPhone and Android classes can now both use the `turnOn` method and the `unlock` method.
+```php
+$iPhone = new iPhone();
+$iPhone->turnOn();
+$iPhone->unlock();
+
+$android = new Android();
+$android->turnOn();
+$android->unlock();
+```
+
+You cannot create an instance of an abstract class. PHP would not know how to use the abstract methods
+so when you try to create an abstract instance you will get an error.
+```php
+$cellPhone = new CellPhone(); // causes an error
+```