There was a problem with the way I added 401 Authentication to PHP iCalendar. To know if I needed to prompt for 401 authentication, I was checking for an error string that contained "401 Authorization Required". It turns out that in versions of PHP earlier than 4.3, the error message didn’t contain this string.
I’ve changed the code to check the version of PHP and if it is earlier than 4.3, any "No such file or directory" error message will result in a prompt for 401 authentication. This is less than ideal and I’m happy to hear any suggestions for improvements. The new code is in tasks 1.8.1rc2 (which I’m about to release).