how to set squid proxy server to get client real ip address

Question

My clients http requests go through Squid proxy server to connect to backend apache web server. A php script in the backend webserver gets clients' real ip address as below:

       if (!empty($_SERVER['HTTP_CLIENT_IP']))
       {
          $ip_address = $_SERVER['HTTP_CLIENT_IP'];
       }
       //whether ip is from proxy
       elseif (!empty($_SERVER['HTTP_X_FORWARDED_FOR']))
       {
         $ip_address = $_SERVER['HTTP_X_FORWARDED_FOR'];
       }
       //whether ip is from remote address
       else
       {
         $ip_address = $_SERVER['REMOTE_ADDR'];
       }

I set the Squid proxy server with the following parameters in order to get the clients real ips:

    via on
    request_header_access  X-Forwarded-For  allow all
    forwarded_for   on
    follow_x_forwarded_for allow localhost

I expected to get the real clients' ips through the parameter $_SERVER['HTTP_X_FORWARDED_FOR']. However, when I checked the values from clients' browsers, only remote_addr has values:

• http_client_ip: undefined
• http_x_forwarded_for: undefined
• remote_addr => squid proxy server ip address

I suspect the issue may locate in my squid proxy server setup, but have no idea which Squid configuration parameters should I change. Appreciated your help in advance.

With Regards

Steve