Constaints

Is there anyway to declare a constant that you can then use within a
postgresql 'session', i am connecting from a PHP based application and
trying to integrate another.

What I want to be able to do is setup a rule on another table so that
whenever a query is run on the table it appends another value to the query.

i.e i want to be able to do:

'SET someconstant an_id=1;'

Then with a rule, running:

SELECT * FROM a_table

would become:

SELECT * FROM a_table WHERE id=a_id

is this possible?

Thanks

Jake