PHP mail special characters in subject field

I would like to insert special characters in the subject of HTML e-mails sent with the PHP mail() function.

I want my subject to look like this:

★ Your new account

I have tried with an HTML entity like ★ (★) or by pasting the symbol directly in my code but that doesn't work either, except on a few e-mail clients.

$to = 'me@example.com';
$subject = '★ Your new account or ★ Your new account';
$message = 'HTML message...';

$headers = 'MIME-Version: 1.0' . "\r\n";
$headers .= 'Content-type: text/html; charset=utf-8' . "\r\n";
$headers .= 'From: Me <me@example.com>' . "\r\n";

mail($to, $subject, $message, $headers);

Any advice on how to get this to work properly? Thank you.