Hm, not that I know of. The biggest thing is to read about PHP's security issues (especially how to avoid SQL injection attacks). After that, it's mostly writing code in it. If you start by writing PHP like you'd write C and then branch out, you can figure it out. Stay away from running anything shell related from your PHP code or anything that manipulates the file system unless you know what you're doing. If you're doing anything that's very critical to your users, I'd probably do it in a different language.
