Learning a new programming language can be a huge undertaking, and learning PHP is no exception. Not only do you have to learn the actual syntax and structure, but there are a number of different subtleties unique to any language, and PHP has its fair share. There are some best practices that you should get in the habit of applying to your work, so you don't have to come back later and break yourself of any bad habits. This is by no means a complete list, but it's a good starting point for PHP beginners.
First and foremost, use the latest version of PHP. This might seem like a no-brainer to everyone, but it's surprising how many people are still using outdated versions of PHP. Just make sure that you check with your hosting provider that they're willing to support the version you use. While all hosts should be supporting PHP 5 by now, they may not be supporting the latest subversion (5.5 at the time of writing)
Next, familiarise yourself with the PHP manual. Another one that might seem like a no-brainer, but most programmers learn from a 'Teach Yourself' guide or similar online tutorial system, and some have never touched the PHP manual in their lives. Give it a shot, and you're almost guaranteed to learn something that was left out of your quick-start guide.
If you haven't already switched to one, try out an integrated development environment, or IDE for short. We recently posted about some of the most popular IDEs, but it's important to find one that feels comfortable to you, so test out some of the available options to find your personal favourite. The simplest improvements over a basic text editor can make you wonder how you ever coded without an IDE.
To save yourself a bunch of headaches while you're learning (and after), make sure you have enabled error reporting. It will save you a huge amount of time trying to sort through code that you're not really familiar with, and you'll be instantly thankful for it. Just make sure that you remember to turn it off before you launch, or your benign users will be confused and any malicious users may have a route into your system.
Finally, make sure you get into the habit of writing your PHP code as cleanly as possible from the beginning. Use naming conventions for your variables that actually mean something, because by the time you're finished your project you'll have lost track of the obscure names you gave to your incremental loops. Indent your code and use plenty of whitespace, which will make sorting through things infinitely simpler and faster, and will make it easier for other programmers to help you out if it turns out you need it.