Auto. change order status after IPN notification

Hello.

I wonder how can I automatically change the order status from "created" to "confirmed" in my database if the IPN notification was received...
I had previously written the corresponding PHP code into the after_end.php file, but I can't be sure that the client will click on the return url link of the paypal thank you page...
So I guess I should put this code into the PHP file which manages the IPN notification, and sends the confirmation e-mail to the administrator.
BUT... I can't find it
Does anyone know where I can find this PHP file ?

Thanks in advance for your help
plastic1st

Hi Nicolas.
Thanks for the answer.
I had just found the solution, and was about to change my topic status
In fact, I hadn't set the 3 status in the paypal plugin.
By default, the 3 status are set to "aucun" in french, so I guess "none" in english, so I just had to set the verified status to "confirmed".
That was so easy...
Thanks anyway for your help.
And thanks for this great component, which make it really easy to manage orders and payments !!