As I said, it could be a tall order
. But PostgreSQL is definitely the way to go. Spend some time in the PostgreSQL forum here and at http://techdocs.postgresql.org/
and you will get more of a feel for why.
BTW, there are a couple of PostgreSQL web-based accounting packages, coded in Perl. At least these could provide a way to get started on a PHP version:
(a very mature, established app)
(I don't know much about this one)
Each of these above apps has some serious security concerns, though, if they are to be run on an external webserver.
As far as I'm concerned, a MySQL-based accounting/commerce package isn't worth the bits it occupies to download. It takes so much more coding in PHP to work around the deficiences such as lack of views, triggers, etc... not to mention the BETA quality of MySQL's referential integrity and transactions, that it's worth the time it takes to learn PostgreSQL. And, it's a much easier step to progress to Oracle, if the need comes up.