PHP and MYSQL for Dynamic Websites, Visual QuickPro Guide, by Larry Ullman, PeachPit PressInstall php and mysql on your local machine (I use wamp server for this) and work through the book a little when you have time. Start reading the CuibeCart files to get a crash course in understanding php. When people have questions on the forum, dig into one of your local installations and see if you can manage a solution.
Pretty much guaranteed to work

You have a hard copy reference/textbook, and hands-on experience reading, understanding and manipulating the code.