Added abstract classes page

code/abstract.php

@@ -0,0 +1,48 @@
+<?php
+
+// 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.
+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.
+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.
+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.
+$iPhone = new iPhone();
+$iPhone->turnOn();
+$iPhone->unlock();
+
+$android = new Android();
+$android->turnOn();
+$android->unlock();
+
+// Lastly, 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.
+$cellPhone = new CellPhone();