RFC - multi database/application user authentification

I have two separate database applications running, one for the initial
Pre-Delivery preparation of new vehicles, and one for the final taxing and
delivery to customers.

These two systems were developed independently but are used by the same
people. Each of these databases has a user table, used for initial
authentication, and also for logging purposes, i.e. who did what action and
when. Each user table has as it's primary key an int4, but these don't match
between the two systems.

I am about to develop a new system to encompass the whole order to delivery
process which will mean the creation of a third system with a third database,
user table and authentication process.

What is the best way to develop an authentication method where one
user-id/password system can be used by three separate databases?

Is it possible to get postgresql to do this cross-database access or will I
have to handle it in the PHP code?
--
Gary Stainburn

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