I would say the most important thing is to learn how the whole process of building a page with PHP works - in that requests come from a client (web browser), hit the web server, get passed through to PHP, which then generates the response that is sent back. A solid understanding of this will ground you in
- why you can't send headers after output has started
- how sessions and cookies work
- how each page should be built in a stateless manner (i.e. deliver whatever the request asks for, don't remember what happened last time, or guess what the user is doing)
- The difference between HTML, PHP, JavaScript and CSS, and more importantly, what each is used for primarily and where the responsibility of each lies.
Once you've got that down, then you should be quite comfortable with writing any app. But unless you've got that down, you'll start mixing things as I've seen many rookies do before now.
