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);
}

第二种办法：

function getip(){
    $ip=false;
    if(!empty($_SERVER['HTTP_CLIENT_IP'])){
        $ip=$_SERVER['HTTP_CLIENT_IP'];
    }
    if(!empty($_SERVER['HTTP_X_FORWARDED_FOR'])){
        $ips=explode (', ', $_SERVER['HTTP_X_FORWARDED_FOR']);
        if($ip){
            array_unshift($ips, $ip);
            $ip=FALSE;
        }
        for ($i=0; $i < count($ips); $i++){
            if(!eregi ('^(10│172.16│192.168).', $ips[$i])){
                $ip=$ips[$i];
                break;
            }
        }
    }
    return ($ip ? $ip : $_SERVER['REMOTE_ADDR']);
}

第三种办法：

function getip() {
    static $realip;
    if(isset($_SERVER)){
        if(isset($_SERVER['HTTP_X_FORWARDED_FOR'])){
            $realip=$_SERVER['HTTP_X_FORWARDED_FOR'];
        }
        else if(isset($_SERVER['HTTP_CLIENT_IP'])){
            $realip=$_SERVER['HTTP_CLIENT_IP'];
        }
        else{
            $realip=$_SERVER['REMOTE_ADDR'];
        }
    }
    else{
        if(getenv('HTTP_X_FORWARDED_FOR')){
            $realip=getenv('HTTP_X_FORWARDED_FOR');
        }
        else if(getenv('HTTP_CLIENT_IP')){
            $realip=getenv('HTTP_CLIENT_IP');
        }
        else{
            $realip=getenv('REMOTE_ADDR');
        }
    }
    return $realip;
}