我想知道也许你应该在爆炸的HTTP_X_FORWARDED_FOR以相反的顺序迭代，因为我的经验是，用户的IP地址结束在逗号分隔的列表的末尾，所以从头的开始，你更有可能得到一个返回的代理的IP地址，这可能仍然允许会话劫持，因为许多用户可能通过该代理。

你已经回答了你自己的问题!：）

function getRealIpAddr() {
    if(!empty($_SERVER['HTTP_CLIENT_IP']))   //Check IP address from shared Internet
    {
        $IPaddress = $_SERVER['HTTP_CLIENT_IP'];
    }
    elseif (!empty($_SERVER['HTTP_X_FORWARDED_FOR']))   //To check IP address is passed from the proxy
    {
        $IPaddress = $_SERVER['HTTP_X_FORWARDED_FOR'];
    }
    else
    {
        $IPaddress = $_SERVER['REMOTE_ADDR'];
    }
    return $IPaddress;
}

源

就像之前有人说的，这里的关键在于你为什么想要存储用户的ip。

我将从我工作的注册系统中给出一个例子，当然解决方案只是为了在这个经常出现在我搜索中的旧讨论中贡献一些东西。

许多php注册库使用ip来限制/锁定基于用户ip的失败尝试。 看看这张表:

-- mysql
DROP TABLE IF EXISTS `attempts`;
CREATE TABLE `attempts` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `ip` varchar(39) NOT NULL, /*<<=====*/
  `expiredate` datetime NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
 -- sqlite
...

然后，当用户尝试登录或任何与重置密码相关的服务时，在开始时调用一个函数:

public function isBlocked() {
      /*
       * used one of the above methods to capture user's ip!!!
       */
      $ip = $this->ip;
      // delete attempts from this ip with 'expiredate' in the past
      $this->deleteAttempts($ip, false);
      $query = $this->dbh->prepare("SELECT count(*) FROM {$this->token->get('table_attempts')} WHERE ip = ?");
      $query->execute(array($ip));
      $attempts = $query->fetchColumn();
      if ($attempts < intval($this->token->get('attempts_before_verify'))) {
         return "allow";
      }
      if ($attempts < intval($this->token->get('attempts_before_ban'))) {
         return "captcha";
      }
      return "block";
   }

例如，$this->令牌->get('attempts_before_ban') === 10和2个用户来相同的ips，就像前面的代码中头部可以被欺骗的情况一样，然后在5次尝试后，两者都被禁止! 更糟糕的是，如果所有用户都来自同一个代理，那么只有前10个用户将被记录，其余的用户将被禁止!

这里的关键是我们需要一个表尝试的唯一索引，我们可以从这样的组合中获得它:

 `ip` varchar(39) NOT NULL,
 `jwt_load varchar(100) NOT NULL

其中jwt_load来自一个遵循json web令牌技术的HTTP cookie，我们只存储加密的有效负载，应该包含每个用户的任意/唯一值。 当然，请求应该修改为:"SELECT count(*) FROM {$this->token->get('table_attempts')} WHERE ip = ?AND jwt_load = ?"并且该类还应该初始化一个私有的$jwt。

然而，即使这样，获取用户的真实IP地址也是不可靠的。他们所需要做的就是使用一个匿名的代理服务器(一个不尊重http_x_forwarded_for, http_forwarded等的头)，你得到的只是他们的代理服务器的IP地址。

然后，您可以查看是否有一个匿名的代理服务器IP地址列表，但没有办法确保这是100%准确的，它所做的最多是让您知道这是一个代理服务器。如果有人很聪明，他们可以欺骗HTTP转发的报头。

假设我不喜欢当地的大学。我知道他们注册了什么IP地址，然后通过做坏事让他们的IP地址在你的网站上被禁止，因为我知道你遵守HTTP转发。这样的例子不胜枚举。

然后，正如您所猜测的那样，还有内部IP地址，例如我前面提到的学院网络。很多人使用10.x.x。x格式。所以你只知道它被转发到一个共享网络。

那我就不多说了，动态IP地址已经是宽带的方式了。所以。即使你获得了一个用户IP地址，预计它最多2 - 3个月就会改变。

我很惊讶没有人提到filter_input，所以这里是Alix Axel的回答浓缩成一行:

function get_ip_address(&$keys = ['HTTP_X_FORWARDED_FOR', 'HTTP_X_FORWARDED', 'HTTP_X_CLUSTER_CLIENT_IP', 'HTTP_FORWARDED_FOR', 'HTTP_FORWARDED', 'HTTP_CLIENT_IP', 'REMOTE_ADDR'])
{
    return empty($keys) || ($ip = filter_input(INPUT_SERVER, array_pop($keys), FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE))? $ip : get_ip_address($keys);
}

来自Symfony的Request类 https://github.com/symfony/symfony/blob/1bd125ec4a01220878b3dbc3ec3156b073996af9/src/Symfony/Component/HttpFoundation/Request.php

const HEADER_FORWARDED = 'forwarded';
const HEADER_CLIENT_IP = 'client_ip';
const HEADER_CLIENT_HOST = 'client_host';
const HEADER_CLIENT_PROTO = 'client_proto';
const HEADER_CLIENT_PORT = 'client_port';

/**
 * Names for headers that can be trusted when
 * using trusted proxies.
 *
 * The FORWARDED header is the standard as of rfc7239.
 *
 * The other headers are non-standard, but widely used
 * by popular reverse proxies (like Apache mod_proxy or Amazon EC2).
 */
protected static $trustedHeaders = array(
    self::HEADER_FORWARDED => 'FORWARDED',
    self::HEADER_CLIENT_IP => 'X_FORWARDED_FOR',
    self::HEADER_CLIENT_HOST => 'X_FORWARDED_HOST',
    self::HEADER_CLIENT_PROTO => 'X_FORWARDED_PROTO',
    self::HEADER_CLIENT_PORT => 'X_FORWARDED_PORT',
);

/**
 * Returns the client IP addresses.
 *
 * In the returned array the most trusted IP address is first, and the
 * least trusted one last. The "real" client IP address is the last one,
 * but this is also the least trusted one. Trusted proxies are stripped.
 *
 * Use this method carefully; you should use getClientIp() instead.
 *
 * @return array The client IP addresses
 *
 * @see getClientIp()
 */
public function getClientIps()
{
    $clientIps = array();
    $ip = $this->server->get('REMOTE_ADDR');
    if (!$this->isFromTrustedProxy()) {
        return array($ip);
    }
    if (self::$trustedHeaders[self::HEADER_FORWARDED] && $this->headers->has(self::$trustedHeaders[self::HEADER_FORWARDED])) {
        $forwardedHeader = $this->headers->get(self::$trustedHeaders[self::HEADER_FORWARDED]);
        preg_match_all('{(for)=("?\[?)([a-z0-9\.:_\-/]*)}', $forwardedHeader, $matches);
        $clientIps = $matches[3];
    } elseif (self::$trustedHeaders[self::HEADER_CLIENT_IP] && $this->headers->has(self::$trustedHeaders[self::HEADER_CLIENT_IP])) {
        $clientIps = array_map('trim', explode(',', $this->headers->get(self::$trustedHeaders[self::HEADER_CLIENT_IP])));
    }
    $clientIps[] = $ip; // Complete the IP chain with the IP the request actually came from
    $firstTrustedIp = null;
    foreach ($clientIps as $key => $clientIp) {
        // Remove port (unfortunately, it does happen)
        if (preg_match('{((?:\d+\.){3}\d+)\:\d+}', $clientIp, $match)) {
            $clientIps[$key] = $clientIp = $match[1];
        }
        if (!filter_var($clientIp, FILTER_VALIDATE_IP)) {
            unset($clientIps[$key]);
        }
        if (IpUtils::checkIp($clientIp, self::$trustedProxies)) {
            unset($clientIps[$key]);
            // Fallback to this when the client IP falls into the range of trusted proxies
            if (null ===  $firstTrustedIp) {
                $firstTrustedIp = $clientIp;
            }
        }
    }
    // Now the IP chain contains only untrusted proxies and the client IP
    return $clientIps ? array_reverse($clientIps) : array($firstTrustedIp);
}