Just have to dive into it. Books are boring but have one handy for a reference. You can pretty much find any topic you need by a Google search. Look at other people's code to get ideas of how to do it.

I don't recommend this but here's how I learned it. My first PHP project was on Rentacoder and I struggled through it until the point the buyer ran out of patience and got his money back through an arbitration. HOWEVER, I learned a lot and did get a partial payment for the product. It got me started but I would not recommend doing it this way unless you like to take big risks.