The NETTUTS.com site has a new tutorial/screencast posted looking at their method for opening up uploaded zip files with PHP:

ThemeForest has a nice feature; It allows the authors to upload zip files containing screenshots of their themes. A script then extracts these files and displays the images accordingly. Though I doubt that the developers used PHP to accomplish this task...that's what we're going to use!

They create a simple form with one field - a file input - and write some basic PHP around it. The script ensures that it was a zip file that was uploaded (based on the mime type) and passes it through a ZipArchive class to do the hard work. This class and the rest of the source can be downloaded from the site.