How to check if client use proxy - PHP Snippet

This php function detect if client is under Proxy or not, if client use proxy will return true.

/***************************************************** 
* 
*    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; 
}