code: protected function getIpAddressFromProxy()
      {
          if ( !$this->useProxy || (isset( $_SERVER[ 'REMOTE_ADDR' ] ) && !in_array( $_SERVER[ 'REMOTE_ADDR' ], $this->trustedProxies ))
          )
          {
              return false;
          }
  
          $header = $this->proxyHeader;
          if ( !isset( $_SERVER[ $header ] ) || empty( $_SERVER[ $header ] ) )
          {
              return false;
          }
  
          // Extract IPs
          $ips = explode( ',', $_SERVER[ $header ] );
          // trim, so we can compare against trusted proxies properly
          $ips = array_map( 'trim', $ips );
          // remove trusted proxy IPs
          $ips = array_diff( $ips, $this->trustedProxies );
  
          // Any left?
          if ( empty( $ips ) )
          {
              return false;
          }
  
          // Since we've removed any known, trusted proxy servers, the right-most
          // address represents the first IP we do not know about -- i.e., we do
          // not know if it is a proxy server, or a client. As such, we treat it
          // as the originating IP.
          // @see http://en.wikipedia.org/wiki/X-Forwarded-For
          $ip = array_pop( $ips );
          return $ip;
      }
docs: Attempt to get the IP address for a proxied client
  
  @see http://tools.ietf.org/html/draft-ietf-appsawg-http-forwarded-10#section-5.2
  @return false|string