One of the best ways to learn a new programming language is to actually have something you need or want to do in the language, and then use online resources to help you figure out how to do it.
Learn the basics with the HelloWorld type programs and then, it's time to do things that matter to you.
If you start small and work your way up, you will learn a lot and feel like you really got something done.
For example, figure out something tiny that you would like to change about your forum. Maybe you want to recognize the 5 most recent members on your site instead of just one. Then, you start reading the index.php code until you can figure out the gist of what's going on and where the code is that relates to the newest member.
Then, it's just a matter of finding code somewhere that does sort of what you want to do, and making it do exactly what you want it to do.
Amy