I'm re-reading your original request and see that you're asking for the MAC address of the client. For some reason, I was thinking that you wanted the MAC address of the server running PHP... that's why I asked whether it was Windows or Linux.
Anyway... PHP is not for client side. There are some environement variables that you can grab from the Web server (such as the client's browser type, IP address, etc.) but to my knowledge, the MAC address isn't one of them.
As LordShryku stated in his post: "it can't purely be done with php". You would need a program to execute on the client side, grab the client's MAC address, assign that to a variable which could be fed to a PHP script.