Add Twitter handle/username to WordPress User Contact Information

The default WordPress user profile page does not have a field for Twitter profile URL.

user_contactmethods filter allows for theme or plugin developers to modify the default set of fields in Contact Information section of wp-admin/profile.php and wp-admin/user-edit.php

The code snippets below illustrate how to add Twitter username as Contact Information field (functions.php) and how to get username for output in your loop or templates (single.php).