use constant instead of string

FactoryMethod/ItalianFactory.php

@@ -18,12 +18,15 @@ class ItalianFactory extends FactoryMethod
     protected function createVehicle($type)
     {
         switch ($type) {
-            case 'cheap' :
+
+            case parent::CHEAP :
                 return new Bicycle();
                 break;
-            case 'fast' :
+
+            case parent::FAST :
                 return new Ferrari();
                 break;
+
             default :
                 throw new \InvalidArgumentException("$type is not a valid vehicle");
         }