What is the best practice for encoding data in PHP before passing it in the subject line of an email?

When passing data in the subject line of an email in PHP, it is important to properly encode the data to ensure it is displayed correctly in the email client. One common method is to use the `mb_encode_mimeheader()` function to encode the subject line using the UTF-8 character encoding. This function will handle special characters and ensure the subject line is properly formatted for email.

$subject = 'Subject with special characters äöü';
$encoded_subject = mb_encode_mimeheader($subject, 'UTF-8');