isolation level and autocommit

I have a question to configuration of PostgreSQL 7.3

If in var/lib/pgsql/data/postgresql.conf the variable autocommit is set to
true, I cannot work with transactions, because all is commited immediately.

I set autocommit to false and set default_transaction_isolation =
'serializable', because this I need for one application. In this
application I set the following command after connect:

SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL SERIALIZABLE

Its work perfect.

But if I work with phpPgAdmin on the database, I have to use "BEGIN" and
"COMMIT" with every action. Is this changeable? It may be ok for me, if it
is only for the one database, but it is now for every pgsql-database on
this server.

I want to have 'autocommit=true' with the one database and the above
behaviour with the other database. How do I realize that?

Thank you for comments in advance.

Manfred Rebentisch