cal_days_in_month is a built in php function
To get these functions to work, you have to compile PHP with --enable-calendar .
The Windows version of PHP has built-in support for this extension. You do not need to load any additional extensions in order to use these functions.
As windows user, I didn't have any problems with that, but it is nice you pointed that out. It's possible to replace cal_days_in_month function with date("t") - equivalent functionality.
I'll update class ASAP.