http://phptherightway.com is a good place to start, if you haven't read it before. It's opinionated, and not necessarily a guide follow down to the letter, but it can put you on the right track.
It seems to be a bit light on advice in places. e.g. the section on comparison operators lightly mentions the difference between == and === but doesn't say anything about when they should be used. As that's one of the traditional PHP gotchas I would have expected more there.
Anyway, thanks; I'll check it out.