请问如何用php获取客户端正确的ip地址？

常用的获取ip方法

    function getip() {
    if (getenv ( "http_client_ip" ) && strcasecmp ( getenv ( "http_client_ip" ), "unknown" ))
        $ip = getenv ( "http_client_ip" );
    else if (getenv ( "http_x_forwarded_for" ) && strcasecmp ( getenv ( "http_x_forwared_for" ), "unknown" ))
        $ip = getenv ( "http_x_forwarded_for" );
    else if (getenv ( "remote_addr" ) && strcasecmp ( getenv ( "remote_addr" ), "unknown" ))
        $ip = getenv ( "remote_addr" );
    else if (isset ( $_SERVER ["REMOTE_ADDR"] ) && $_SERVER ["REMOTE_ADDR"] && strcasecmp ( $_SERVER ["REMOTE_ADDR"], "unknown" ))
        $ip = $_SERVER ["REMOTE_ADDR"];
    else
        $ip = "unknown";
    return ($ip);
}

但用代理后取得的ip就不对了，请问该肿么办呢？