In Drupal, if you have a content type and you want to change the output of any field to something else you can use field formatter with field API. This enables a display settings for any fields in a content type and where you can manage the output as you wish. This article shares how to create a field formatter for your Drupal content types. There are some contributed modules for custom field formatters. But if you really want to create or learn about creating a field formatter programmatically, just go through this article