Adding Order ID to mail subject

[walter6]

Hello,

 

could somebody please tell how to add the order-id to the mail subject being send to the customer after making an order?

 

I've tried adding

.sprintf("%06d", $order->id)

(just like it is done in mailalerts.php)

in classes/Mail.php but it just adds 000000 (zeros) instead of the actual number.

 

Thanks.