Custom security through functions.

Hi

I wish it add my own layer of security based primarily on username and
password for a PHP application. Basically i wish to perform
authentication through my own PL/PGSQL function that should set some
kind of session only variable. This variable should then be used as
security token in the following functions (they work as a middle layer
between application and the actual tables performing the actual queries)
be checked for simple validity. Functions are executed with function
authors privileges only.

My problem is that i havent identified what kind of variable type (if
any) i can use that a session may not just set on it's own (can i block
this with ACL?). Perhaps some kind of temporary table? Are there any
kinds that would expire as soon as session is over?

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Sten Daniel Sørsdal