What could be causing the issue of a strange exclamation mark appearing in PHP emails?

The strange exclamation mark appearing in PHP emails could be caused by encoding issues when sending special characters in the email content. To solve this issue, you can use the PHP `mb_encode_mimeheader()` function to properly encode the email subject before sending it.

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

// Send email with properly encoded subject
mail('recipient@example.com', $subject, 'Email content', 'From: sender@example.com');