BadBlue is a free, small footprint web server that supports PHP, CGI and ISAPI. The newly released version supports peer-to-peer file sharing, including live sharing of Excel spreadsheets and Word documents using only a browser.
If you're looking for a very fast way to learn, develop and/or deploy PHP applications (especially mobile/wireless apps), this is an excellent way to go. With its built-in file sharing, it's well suited for mobile app serving using PHP as the platform (e.g., you can build powerful sales force automation and field service applications using PHP and BadBlue, incorporating live spreadsheet and document sharing).
Because of its small size (the server itself is about 65K) and ease of use, it can be downloaded, installed and configured to run PHP in about a minute.