I'd say one of the most important things is that you need to be motivated. Put simply, if you're motivated, you'll do the job properly. If you're doing it more for the money and don't really like programming or computers, then you're pretty much wasting your time. And even if you're not motivated, at least try to get a good understanding of web programming so you're not the one making errors.
Often people start big projects before they have the necessary understanding of the language, program design, and as the last poster pointed out, it's all too easy for a new coder to develop a software full of security holes so make sure you know the most common security flaws in PHP apps and how to avoid them. You don't even need to buy books for this, the internet has plenty of PHP tutorials. In fact, I've written/am writing some PHP tutorials that you can see in my sig.
An important point: The PHP Manual is your friend. Make sure you know what parameters a function takes, any version-specific behaviour and what it returns. This is all documented thoroughly in the manual so don't let it go to waste. You could also try helping out other users here and on other forums. By seeing how others go wrong you can help yourself not to make these errors. Helping others also boosts your understanding.
Another thing you can do is develop a simple open source web application. This will help you to learn and understand programming on the web, and it provides a good start to your portfolio. Companies won't be interested if you haven't developed anything before. A good open source application can demonstrate your skills and also how you fit in with the PHP/web programming community.
I wish you luck and I'm sure you'll do a great job 🙂
