Personally, I found the book "Build your own database driven website using PHP and MySQL" extremely helpful.
Other than that, the one thing I found hardest to get used to with PHP is how relaxed it is, compared with any other language I've ever used. By that I mean no types, flexibility about syntax and punctuation. Personally I think that's a good thing, but I also know that it probably encourages pretty bad behavior.
Here's one other tip I have: try to use something like the DRY principle -- i.e., you'll find yourself writing the same little (or big) bits of code over and over again -- make them into functions as early as you can in the process of coding, and life will be a lot easier later on.
