Given the fact that this extension uses Notes' C API interface, you're stuck with Windows. If you're going to go that route, you can use Notes' COM interface just as easily. You won't have to use this extension, and you'll get a much richer functionality.
However, if you would rather stay cross-platform, a fully supported technique is to use XML. You access Domino via file_get_contents or fopen (or similar method), and Domino returns XML, which you then parse within PHP. It is cross-platform, requires no extensions, and is the recommended solution.
Introduction to these techniques (including using the above extension) is covered here. Restore this back into a single line (broken because of wordwrap): http://www-128.ibm.com/developerworks/lotus/