プロキシの背後で実際の訪問者の IP アドレスを取得するにはどうすればよいですか?

<code class="php">function getUserIP()
{
    // Handle CloudFlare network
    if (isset($_SERVER["HTTP_CF_CONNECTING_IP"])) {
        $_SERVER['REMOTE_ADDR'] = $_SERVER["HTTP_CF_CONNECTING_IP"];
        $_SERVER['HTTP_CLIENT_IP'] = $_SERVER["HTTP_CF_CONNECTING_IP"];
    }

    $client  = @$_SERVER['HTTP_CLIENT_IP'];
    $forward = @$_SERVER['HTTP_X_FORWARDED_FOR'];
    $remote  = $_SERVER['REMOTE_ADDR'];

    if(filter_var($client, FILTER_VALIDATE_IP))
    {
        $ip = $client;
    }
    elseif(filter_var($forward, FILTER_VALIDATE_IP))
    {
        $ip = $forward;
    }
    else
    {
        $ip = $remote;
    }

    return $ip;
}

$user_ip = getUserIP();

echo $user_ip; // Output real visitor IP address</code>