Add createdAt property + setter and getter

"createdAt" property and "setCreatedAt" is not defined on section "Using Callbacks to Serialize Properties with Object Instances" so the PHP Fatal error is triggered: Call to undefined method Person::setCreatedAt()

Author: liviubalan

components/serializer.rst

@@ -72,6 +72,7 @@ exists in your project::
         private $age;
         private $name;
         private $sportsman;
+        private $createdAt;
 
         // Getters
         public function getName()
@@ -84,6 +85,11 @@ exists in your project::
             return $this->age;
         }
 
+        public function getCreatedAt()
+        {
+            return $this->createdAt;
+        }
+
         // Issers
         public function isSportsman()
         {
@@ -105,6 +111,11 @@ exists in your project::
         {
             $this->sportsman = $sportsman;
         }
+
+        public function setCreatedAt($createdAt)
+        {
+            $this->createdAt = $createdAt;
+        }
     }
 
 Now, if you want to serialize this object into JSON, you only need to