How to get MAC address of client using PHP?

echo GetMAC();

function GetMAC(){
    ob_start();
    system('getmac');
    $Content = ob_get_contents();
    ob_clean();
    return substr($Content, strpos($Content,'\\')-20, 17);
}

Above will basically execute the getmac program and parse its console-output, resulting to MAC-address of the server (and/or where ever PHP is installed and running on).

The MAC address (the low-level local network interface address) does not survive hops through IP routers. You can't find the client MAC address from a remote server.

In a local subnet, the MAC addresses are mapped to IP addresses through the ARP system. Interfaces on the local net know how to map IP addresses to MAC addresses. However, when your packets have been routed on the local subnet to (and through) the gateway out to the "real" Internet, the originating MAC address is lost. Simplistically, each subnet-to-subnet hop of your packets involve the same sort of IP-to-MAC mapping for local routing in each subnet.

Here's a possible way to do it:

$string=exec('getmac');
$mac=substr($string, 0, 17); 
echo $mac;