Ajax Availability Calendar

Hello,
I receive this error when log out any user:

Fatal error: Call to undefined function session_is_registered() in C:\xampp\htdocs\...\ac-admin\admin-logout.php on line 9


Could you help me, please?
Regards!

Thank you very much, Chris.
Regards,
JC

Hello, first of all: thank you very much for your script. Well, the problem is this: I see the warning "Deprecated: function.gmmktime... bla bla bla" over each calendar in my web page (4 calendars = 4 php warnings). Probably not a bug, however it would need a review. Kind regards from Arge...