PHP code to determine whether the user is using a proxy

 /*****************************************************
*
*    isProxy
*    check whether client is under Proxy or not
*    $_SERVER['HTTP_X_FORWARDED_FOR'] is pivotal here
*    if client is under Proxy, returns true
*    if client is not under Proxy, returns false
*
*****************************************************/

function isProxy(){
 return (!empty($_SERVER['HTTP_X_FORWARDED_FOR'])) ? true : false;
}
?>

PHP