I thought it’d be cool to gather everyone’s collective brains and make a list of things we consider indispensable for PHP (or anything else, I’d like to see separate threads on separate topics if this is successful).
It can be anything from low level nitty gritty stuff to high level theory type things. The only thing I ask is that you post at least one link to a thoughtful article on the topic.
Now we all become collectively awesomer.