Oh, they call that an extention. Well. There's a serial/serial8 type in Postgres, which implicitly creates a sequence and you can use the functions nextval() and currval(). So a definition of e.g. "cur_id serial8 PRIMARY KEY" should handle this.
Or just serial, which would be considerably faster than serial8, unless you think that you will end up with more than 2147483648 rows (2^31).
But the issue is not just the database--it's how to implement the wfInsertID() function in the wiki code. PHP's Postgres functions don't have anything like it, so you'll have to do something like get_last_oid() and then retrieve the serial field value by OID.