The deal with Promethazine is that it induces a slight delirium like DPH does, but in a different fashion. The concept of "leaning" was made popular by, none other than, Lil Wayne due to it appearing in a shit ton of his music (don't ask me which songs, I don't listen to it, but have been made to hear it by a friend that loves codeine / promethazine combos). The trick is to take some extra oxycodone so the higher ratio of prometh in the "sizzurp" doesn't cause too much head-fuck while still getting the appropriate opiate high.
Anyway, don't do this. He got sloppy as hell for a few weeks and his memory was laughable at best (very annoying when dealing with money / actually it was just fucking annoying dealing with him at all...).