Thomas Buck has put together what he calls "Tom's PHP Commandments" (nine of them) that developers should follow when developing applications with the language. Among his suggestions are things like:

• ALWAYS do the simplest thing that will work
• NEVER trust anything that comes from the user
• NEVER use include for controlling logic
• NEVER create a file of useful functions, even if it's called helpers.php

There's also been some discussion about this list over on Reddit - you can comment on it over there if you agree/disagree with some of his points.