In this example, the function session_start() is called to start a session. The session ID is then added to the form as a hidden input field. When the form is submitted, the session ID will be included in the $_POST array and can be used to retrieve session data. Library used: It is likely that the add_hidden function is part of a larger PHP library or framework, such as Laravel or Symfony. However, without additional context or code, it is difficult to determine the specific library used.