Use this function to get the client MAC address:
function GetClientMac(){
$macAddr=false;
$arp=`arp -n`;
$lines=explode("\n", $arp);
foreach($lines as $line){
$cols=preg_split('/\s+/', trim($line));
if ($cols[0]==$_SERVER['REMOTE_ADDR']){
$macAddr=$cols[2];
}
}
return $macAddr;
}