PHP code to determine whether the user is using a proxy

<?php
/*****************************************************
*
*    isProxy
*    check whether client is under Proxy or not
*    $_SERVER[&#39;HTTP_X_FORWARDED_FOR&#39;] is pivotal here
*    if client is under Proxy, returns true
*    if client is not under Proxy, returns false
*    
*****************************************************/
 
function isProxy(){
    return (!empty($_SERVER[&#39;HTTP_X_FORWARDED_FOR&#39;])) ? true : false;
}
?>