OO & MySQL & calculating

[Squarry]

Hi!

I´d like to do the following:

In a MySQL database there are some values of billings stored (Name, Street, amount,...). Now I want to take some values and calculate them (algo modulo 10). Then the result should be written into a writer document and also be written into another field in the database (rnumber). And all this as a mail merge.
The goal is to have the possibility to print payment slips because my application is not able to do this. So i have to print about 50 slips each time.

The slips contain the adress, amount, a special number that i have to calculate out of some values and some kind of a checksum with special chars. it´s a combination of calculating and combining some values.

database connection is no problem and i can read everything that i want. but how can i calculate with the values? now i have them in a spreadsheet, but there i can also not do a mail merge. and how can i automatically for each customer write the calculated numbers into the database?

thanks a lot!

Squarry