How to identify which column in a table is the primary key

I am using Postgresql 7.2.4 and php 4.2.2 to write some generic
functions to access and manipulate all tables in a database.

I have been looking for a way to identify the primary key defined in a
table. The closest I have found is a filed in the pg_class table that
is a boolean that tells me if that table has or every had a primary
key.

So given the table name is there a way to identify the column defined as
the primary key for that table?

--
Scot L. Harris <webid(at)cfl(dot)rr(dot)com>