Whisper should run on PHP 4.38 and up. There are no additional requirements. Whisper does not rely on a database, and works on both Windows and UNIX-based operating systems (including Mac OS X).
Specific features’ requirements:
The File Manager plugin requires a UNIX-based operating system. If you are running Whisper on Windows, you should delete file_manager.php from the plugins directory. Attempting to run the File Manager on Windows will show you an error, although it will not otherwise cause problems.