server variables ?

Hi,

are there something like variables on the server-side that can be set by
the client?

e.g. I'd like to store the user-id who created or updated a record.
I've allready got a trigger that sets a timestamp whenever a record is
updated. It was cute if I could let this triggers store the user-id, too.

I'd like to open a connection when the client application starts.
The client should set a variable  userid  within this connection and
every trigger would know who uses this connection right now.

BTW ... How would I use this variable as a DEFAULT value when it should
be set just at the creation time of the record?
something like a collumn:
...
created_by_fk   integer   not null   default userid,
...

How could I use such a variables value in views?