title: How can I get client's computers mac address or identifier of a machine
body: <p>I need to stop multiple login from same computer even for different user. This is why need client machine's unique identifier or mac address. How can achieve this? I have tried following but not work. Its return <code>NULL</code></p>
  
  <pre><code>$ip  = $_SERVER['REMOTE_ADDR'];
  $x = shell_exec('arp -a ' . escapeshellarg($ip));
  </code></pre>
  
  <p>I need this feature that will work on all browser. Any Idea?</p>
  
  <p>I have seen some solution only work in IE. But how can get solution in all modern browsers?</p>