problems?

I have the following scenario: I have this timeclock program that my employees access by logging into and the program pretty much allows them to clock-in and clock-out. The problem is that I noticed some of the employees were clocking-in and clocking-out from their home. The main problem is that the machine at work that the employees use to access this program does not have a static ip address. Is there any way to access the mac address in php. Any ideas? thanks, Dale