retrieving visitor's MAC address

[florescut]

I know visitor's IP addresses can be detected in PHP but how about computer's MAC address which is unique for each network card.
It is possible to retrieve with PHP a visitor's MAC address?

Also, how can be restricted the multiple access from one computer to an online voting system?

Any advice is welcome. Thank you so much.

[ergophobe]

Did you try to google on

php MAC address

That turned up some promising stuff, but since I know zero about this, I couldn't possibly say whether any of it is any good. Sorry.