<?php   //方法1：
$ip = $_SERVER["REMOTE_ADDR"];
echo $ip;

//方法2：
$user_IP = ($_SERVER["HTTP_VIA"]) ? $_SERVER["HTTP_X_FORWARDED_FOR"] : $_SERVER["REMOTE_ADDR"];
$user_IP = ($user_IP) ? $user_IP : $_SERVER["REMOTE_ADDR"];
echo $user_IP;

//方法3：
function getRealIp()
{
    $ip=false;
    if(!empty($_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']);
}
echo getRealIp();

//方法4：
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"];
}
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";
}
echo $ip ;

//方法5：
if(getenv('HTTP_CLIENT_IP')) {
    $onlineip = getenv('HTTP_CLIENT_IP');
} elseif(getenv('HTTP_X_FORWARDED_FOR')) {
    $onlineip = getenv('HTTP_X_FORWARDED_FOR');
} elseif(getenv('REMOTE_ADDR')) {
    $onlineip = getenv('REMOTE_ADDR');
} else {
    $onlineip = $HTTP_SERVER_VARS['REMOTE_ADDR'];
}
echo $onlineip;