What about this? Get the user's MAC! I think it's a quite hardcore, but it works.
ob_start();
system('ipconfig /all');
$mycom=ob_get_contents();
ob_clean();
$findme = "Physical";
$pmac = strpos($mycom, $findme);
$mac=substr($mycom,($pmac+36),17);
echo $mac;
?>
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