On the Web Development Blog, there's a quick tutorial on creating dynamic zip files via a helpful little class:

For a future project I needed these days some easy to use zip or gzip class to create a zip file from files / folders inside a specified directory. [...] I tested two [other] scripts before and must say that this script works great for single files if you add them manually. To compress a whole directory with an unknown number of files into one zip file I created some class extension to get this job done.

In his class, the get_file_from_folder method is called with the directory and the file to load them into. This loops through the files, pulling them all (directories and all) into the new zip file. An example of the class' use is also included.