像下面这样?

if (($ip=filter_input(INPUT_SERVER, 'REMOTE_ADDR', validate_ip)) === false or empty($ip)) {
    exit;
}
echo $ip;

PS

if (($ip=filter_input(INPUT_SERVER, 'REMOTE_ADDR', FILTER_VALIDATE_IP|FILTER_FLAG_NO_PRIV_RANGE|FILTER_FLAG_NO_RES_RANGE)) === false) {
    header('HTTP/1.0 400 Bad Request');
    exit;
}

所有以'HTTP_'或'X-'开头的报头都可能被欺骗，分别由用户定义。如果你想要跟踪，可以使用cookie等。

它应该包含在$_SERVER['REMOTE_ADDR']变量中。

快速解决方案(无错误)

function getClientIP():string
{
    $keys=array('HTTP_CLIENT_IP','HTTP_X_FORWARDED_FOR','HTTP_X_FORWARDED','HTTP_FORWARDED_FOR','HTTP_FORWARDED','REMOTE_ADDR');
    foreach($keys as $k)
    {
        if (!empty($_SERVER[$k]) && filter_var($_SERVER[$k], FILTER_VALIDATE_IP))
        {
            return $_SERVER[$k];
        }
    }
    return "UNKNOWN";
}

function get_client_ip()
{
    foreach (array(
                'HTTP_CLIENT_IP',
                'HTTP_X_FORWARDED_FOR',
                'HTTP_X_FORWARDED',
                'HTTP_X_CLUSTER_CLIENT_IP',
                'HTTP_FORWARDED_FOR',
                'HTTP_FORWARDED',
                'REMOTE_ADDR') as $key) {
        if (array_key_exists($key, $_SERVER)) {
            foreach (explode(',', $_SERVER[$key]) as $ip) {
                $ip = trim($ip);
                if ((bool) filter_var($ip, FILTER_VALIDATE_IP,
                                FILTER_FLAG_IPV4 |
                                FILTER_FLAG_NO_PRIV_RANGE |
                                FILTER_FLAG_NO_RES_RANGE)) {
                    return $ip;
                }
            }
        }
    }
    return null;
}

或压缩版:

function get_ip() {
    foreach (array('HTTP_CLIENT_IP', 'HTTP_X_FORWARDED_FOR', 'HTTP_X_FORWARDED', 'HTTP_X_CLUSTER_CLIENT_IP', 'HTTP_FORWARDED_FOR', 'HTTP_FORWARDED', 'REMOTE_ADDR') as $key) {
        if (array_key_exists($key, $_SERVER) === true) {
            foreach (array_map('trim', explode(',', $_SERVER[$key])) as $ip) {
                if (filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE) !== false) {
                    return $ip;
                }
            }
        }
    }
}

我最喜欢的解决方案是Zend Framework 2使用的方式。它还考虑$_SERVER属性HTTP_X_FORWARDED_FOR, HTTP_CLIENT_IP, REMOTE_ADDR，但是它声明了一个类来设置一些可信代理，并且它返回一个IP地址而不是一个数组。我认为这是最接近它的解决方案:

class RemoteAddress
{
    /**
     * Whether to use proxy addresses or not.
     *
     * As default this setting is disabled - IP address is mostly needed to increase
     * security. HTTP_* are not reliable since can easily be spoofed. It can be enabled
     * just for more flexibility, but if user uses proxy to connect to trusted services
     * it's his/her own risk, only reliable field for IP address is $_SERVER['REMOTE_ADDR'].
     *
     * @var bool
     */
    protected $useProxy = false;

    /**
     * List of trusted proxy IP addresses
     *
     * @var array
     */
    protected $trustedProxies = array();

    /**
     * HTTP header to introspect for proxies
     *
     * @var string
     */
    protected $proxyHeader = 'HTTP_X_FORWARDED_FOR';

    // [...]

    /**
     * Returns client IP address.
     *
     * @return string IP address.
     */
    public function getIpAddress()
    {
        $ip = $this->getIpAddressFromProxy();
        if ($ip) {
            return $ip;
        }

        // direct IP address
        if (isset($_SERVER['REMOTE_ADDR'])) {
            return $_SERVER['REMOTE_ADDR'];
        }

        return '';
    }

    /**
     * Attempt to get the IP address for a proxied client
     *
     * @see http://tools.ietf.org/html/draft-ietf-appsawg-http-forwarded-10#section-5.2
     * @return false|string
     */
    protected function getIpAddressFromProxy()
    {
        if (!$this->useProxy
            || (isset($_SERVER['REMOTE_ADDR']) && !in_array($_SERVER['REMOTE_ADDR'], $this->trustedProxies))
        ) {
            return false;
        }

        $header = $this->proxyHeader;
        if (!isset($_SERVER[$header]) || empty($_SERVER[$header])) {
            return false;
        }

        // Extract IPs
        $ips = explode(',', $_SERVER[$header]);
        // trim, so we can compare against trusted proxies properly
        $ips = array_map('trim', $ips);
        // remove trusted proxy IPs
        $ips = array_diff($ips, $this->trustedProxies);

        // Any left?
        if (empty($ips)) {
            return false;
        }

        // Since we've removed any known, trusted proxy servers, the right-most
        // address represents the first IP we do not know about -- i.e., we do
        // not know if it is a proxy server, or a client. As such, we treat it
        // as the originating IP.
        // @see http://en.wikipedia.org/wiki/X-Forwarded-For
        $ip = array_pop($ips);
        return $ip;
    }

    // [...]
}

在这里查看完整代码: https://raw.githubusercontent.com/zendframework/zend-http/master/src/PhpEnvironment/RemoteAddress.php

正如所有其他人所说，你可以使用$_SERVER['REMOTE_ADDR'];获取客户端IP地址。

另外，如果你需要一个用户的更多信息，你可以使用这个:

<?php
    $ip = '0.0.0.0';
    $ip = $_SERVER['REMOTE_ADDR'];
    $clientDetails = json_decode(file_get_contents("http://ipinfo.io/$ip/json"));
    echo "You're logged in from: <b>" . $clientDetails->country . "</b>";
?>

客户端更具体的信息放在$clientDetails中。 $clientDetails->PostalCode/hostname/region/loc…

我用的是ipinfo。IO来获取额外的信息。