$response->setResponseSubject('Thank you for your inquiry!');
$response->setResponseSubject('Error: Invalid input');This code sets the subject line of the response to "Error: Invalid input". This could be used in an email response to a user who has entered invalid input on a web form. The package library for this code could be a PHP form validation library such as Zend\Validator or Symfony\Validator. In general, the setResponseSubject method is commonly used in PHP libraries that handle email responses, web forms, and other types of user input validation. The specific package library used will depend on the application's requirements and framework.