how to trigger incomming mail

hi, I'm building a online shop where people are able to pay online. The pay part is done by a external company who sends mail to a specifeid mail address when orders are placed and when orders are succesfully payed. both mails contains a order nummer in subject. now what I want is to automaticly match those 2 mail so I can write in a mysql database that the order is payed. I'd made a mail address payment@.... on this mail I want to trigger mail incomming on this adres so I can start a php script that filters this standaard mail getting the order code and put in in a database with status incompleet and when another mail arrives on the payment address, filter order nummer again so I can look in the database to see if it there, to compleet the payment kind regards Jeroen Olthof