How to sum monetary variables

I'm building a table (which is a report that has to be printed) with a
bunch of items (up to 300 in some cases) that have unitary price
(stored in a numeric(9,2) field), how many there are, and the total
price for each item. At the end of the table there is a total of all
the items.

The app is running on PHP and PG is the backend.

The question is, how do I get the total of everything?

Running it on PHP gives one value, doing a sum() on the backend gives
another, and I'm starting to notice that even using python as a
calculator gives me errors (big ones). Right now I'm doing the maths
by hand to find out who has the biggest error, or if any is 100%
accurate.

Any ideas?

--
Martín Marqués
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DBA, Programador, Administrador