$to = 'example@example.com';
$subject = 'Subject with non ASCII ó¿¡á';
$message = 'Message with non ASCII ó¿¡á';
$headers = 'From: example@example.com'."\r\n"
.'Content-Type: text/plain; charset=utf-8'."\r\n";
mail($to, '=?utf-8?B?'.base64_encode($subject).'?=', $message, $headers);