On systems that use PHPLIB, there is a conflict running setup.php because both mycalendar and PHPLIB has session.inc.
A correct installation of PHPLIB requires that you use the prepend facility in PHP (php3.inc) to prepend the file prepend.php3.
It is prepend.php3 that loads session.inc.
The easy solution, other than renaming the session.inc file in myphpcalendar is to create an empty (dummy) prepend.php3 file in the PHP directory of the calendar software. This assumes that your PHP include path looks in the current directory (.) first. This means that the PHPLIB will NOT be loaded when the calendar files are accessed, but the rest of the system's software is unaffected.
A rather important consideration for large installations!