Hello David, the way I learned PHP was by starting off with a small sendmail script from a website contact form and gradually expanding it into something greater.
I worked on a template email system and picked at the code and added to it until I pretty much got what I wanted.
That is quite a slow way to learn, but I found it gave me a good understanding of the language.
As you wish to add new features or understand the existing ones within the code, you will probably find the PHP manual is essential.
It can be quite daunting at first, but if you don't understand something, persevere. If you still don't understand something, search the web for the answer and if that still doesn't help, ask about it on this forum.
You will find that much of your time is spent debugging syntax errors. Usually you'll find you missed a semi-colon or forgot to close a bracket or something along those lines. Finding where you 'break' your code is a slow process at first, but will also become a lot easier.
The main thing is never to give up. There are plenty of resources available for learning PHP. If you get stuck and have a particular question, feel free to ask this forum.
I started at http://www.tuxradar.com/practicalphp and I learned quite alot. I found that i did need to go back over a few things and research a little deeper (Google is your friend) but it a good starting point.