Personally, classes are over-rated. Your best bet would be to learn PHP from a book (O'Reilly PHP Cook-book, SAMS Teach Yourself * in XX Days) are some that I've read. They're pretty good. Plus, it's a lot cheaper:
SAMS Teach Yourself PHP, MySQL, and Apache All-In-One
O'Reilly's PHP Cookbook <-- That's more of a book with problems and solutions, but a good reference none the less.
PHP & MySQL For Dummies
Why do I personally like books? Because you can get the SAME standard information from a book as you can from a teach for 1/100 the cost. Yes, teachers could actually give you hints and pointers and shortcuts, but not until after you know the basics. And that is what this community should be. A place to get pointers, tips and shortcuts.
If you want to learn phpBB (the forum script) then I suggest you learn PHP, MySQL, how they interact, and classes and templating with PHP. Once you get that, examine phpBB and do as you wish. You could also ask for their help on their forums as well.
~Brett
