php获取客户端IP

if ($HTTP_SERVER_VARS["HTTP_X_FORWARDED_FOR"])
    $ip = $HTTP_SERVER_VARS["HTTP_X_FORWARDED_FOR"];
elseif ($HTTP_SERVER_VARS["HTTP_CLIENT_IP"])
    $ip = $HTTP_SERVER_VARS["HTTP_CLIENT_IP"];
elseif ($HTTP_SERVER_VARS["REMOTE_ADDR"])
    $ip = $HTTP_SERVER_VARS["REMOTE_ADDR"]; //this is right
elseif (getenv("HTTP_X_FORWARDED_FOR"))
    $ip = getenv("HTTP_X_FORWARDED_FOR");
elseif (getenv("HTTP_CLIENT_IP"))
    $ip = getenv("HTTP_CLIENT_IP");
elseif (getenv("REMOTE_ADDR"))
    $ip = getenv("REMOTE_ADDR");
else
    $ip = "Unknown";