On DZone.com today John Esposito has posted about three guides that want to help you improve your PHP development skills (including a forum post and two articles).

Sometimes, then, improving your generic 'programming brain' will help you improve your facility with a particular language. At other times, it's more important to learn the nuances of a language, paying close attention to the kind of applications the language is used for. [...] For improving your PHP, then, you can do two things: become a better programmer; and understand PHP more finely, more deeply.

The three guides share a lot of the same concepts in common - naming conventions, separation of functionality, DRY (don't repeat yourself), testing code, etc.