Update on existence?

Hello all,

I have a table called sessions, with session ids. Each session also
has a last_access timestamp and some other data. Is is possible (using
PL/pgSQL) to implement something along the line of:

------------------
select id, username, settings from sessions where id='foobar'
If a result was found
update sessions set last_access=current_timestamp where id='foobar'
------------------

Obviously, I want to access the id, username and settings columns if
they are found.

In other words: Is it possible to make something which behaves almost
like a normal "select", but which has some hidden internal magic as
described above?

--
Kind regards,
Jan Danielsson