Hep bilgi istedik, birazda paylaşalım :)

function getMAC($comp)
{
     $IP = gethostbyname($comp);
     exec("ping -n 1 $comp");
     $haystack = shell_exec("arp -a");
     $haystack = " ".$haystack;
     if (strpos($haystack, $IP) == false)
     {
          return "Cannot Obtain MAC";
     }
     else 
    {
          $pos = strpos($haystack, $IP);
          $haystack = substr($haystack, $pos);
          $parts = explode("\n",$haystack);

          foreach ($parts as $part)
          {
               $part = " ".$part;
               if (strpos($part, $IP) == false) 
               {
                    # Do nothing. IP is not here
               }
              else 
              {
                    # IP is on this line
                    $subparts = explode("    ",$part);
                    foreach ($subparts as $sub)
                    {
                        if (substr($sub,0,2) == "00") # All MACs start with "00"?
                        {  
#NOTE: This following part just breaks the MAC up to eliminate the "-"s
                             $macparts = explode("-",$sub); 
                             $computer["mac"] = "";
                             foreach ($macparts as $macp) 
                                   $computer["mac"].=$macp;
                        }
                    }
               }
          }
    return "Oops. Couldn't find the IP for ".$comp;
}
return "Cannot Obtain MAC";
}