This was renamed from ENCRYPTION_ALGO in version 3.3.0
Once you have this set, be careful to change it, as you may lock yourself out from your phpList installation. You may still be able to use the “Forgot password” functionality to get back in.
If you upgraded from older versions of phpList, you may want to set this to “md5”. phpList tries to figure that out automatically, but this may not always work.