PHP getenv() 获取IP

<?php
    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_FORWARDED_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);
    }
    $ip = GetIP();
    echo $ip;
?>