The majority of functions are defined in files contained in the /includes/functions/ folder. And the majority of classes are defined in files contained in the /includes/classes/ folder.

If you're using an integrated development environment (IDE) such as Aptana or Eclipse or phpStorm, you'll find that the search capability will save you tons of time and let you find the function you seek quite easily. While not really designed for PHP, Dreamweaver is almost next in line after those I just mentioned. http://www.zen-cart.com/wiki/index.php/Useful_Tools