Paypal Integration Script Fix

I have a website that takes donations. Donors fill in a form and when they submit, their details are recorded in a MySQL database with the payment status field set to "Not Complete" and then they are taken to Paypal. They pay in Paypal and if they click "Return to Merchant Account" they return to my website and the payment status is automatically changed to "Complete". This is what I want and it all works OK. The Project is to fix a problem: If the client completes the form on my website, they are taken to Paypal (and payment status in my database is set to "not complete". If they then pay and then do not return to my website (eg they go to another website) - the payment status field in my database is not updated to "Complete". This means that the Pasypal record and my record are not synchronised. My database shows "Not Complete" but Paypal shows as paid. I would like a good PHP programmer with excellent working knowledge of Paypal integration to fix this for me so that when the donor pays in Paypal and then does not return to my website, paypal posts a message to my site which is handled by a script to update my database to show the payment as "Complete". This should happen automatically - no human involvement. I am only looking for people who have done PHP projects and know how to integrate with Paypal.