Even if you are a complete beginner with no PHP knowledge, this should be the first one you look at - great, no complaints at all.

in an afternoon as complete ab-initio in PHP, together with the most cursory look at the tutorials and help, and looking at some sample freeeware PHp scripts it helped me write a price calculating website page, and debugged the java paypal buttons, and I was taking cash orders online same day.
- amazed that a program so easy to use and bug free is freely available. Great.