On the Internet Super Hero blog, there's a post that looks at how PDO is implemented in PHP and wonders if the PDO::FETCH_UNIQUE constant is broken (as something to fix before even considering the move to PDO2).

I spent quite a lot of time comparing the different behaviours of the various drivers in the hope I could find out how PDO drivers are supposed to work. The PDO documentation and the specification do not cover each and every detail. PDO really needs some love...

He picks out the FETCH_UNIQUE constant as one that needs a little work and tries to track down exactly what it's doing. One issue he found was that it requires combination with other flags to make the unique part of it work correctly (like FETCH_OBJ or FETCH_COLUMN). He works through several examples, both ones that apply the unique call and others that don't, comparing the results.