获得用户的真实IP地址

/**
 * 获得用户的真实IP地址
 *
 * @access  public
 * @return  string
 */
if (!function_exists('get_real_ip'))
{
   function get_real_ip()
   {
      static $realip = NULL;

      if ($realip !== NULL)
      {
         return $realip;
      }

      if (isset($_SERVER))
      {
         if (isset($_SERVER['HTTP_X_FORWARDED_FOR']))
         {
            $arr = explode(',', $_SERVER['HTTP_X_FORWARDED_FOR']);

            /* 取X-Forwarded-For中第一个非unknown的有效IP字符串 */
            foreach ($arr AS $ip)
            {
               $ip = trim($ip);

               if ($ip != 'unknown')
               {
                  $realip = $ip;

                  break;
               }
            }
         }
         elseif (isset($_SERVER['HTTP_CLIENT_IP']))
         {
            $realip = $_SERVER['HTTP_CLIENT_IP'];
         }
         else
         {
            if (isset($_SERVER['REMOTE_ADDR']))
            {
               $realip = $_SERVER['REMOTE_ADDR'];
            }
            else
            {
               $realip = '0.0.0.0';
            }
         }
      }
      else
      {
         if (getenv('HTTP_X_FORWARDED_FOR'))
         {
            $realip = getenv('HTTP_X_FORWARDED_FOR');
         }
         elseif (getenv('HTTP_CLIENT_IP'))
         {
            $realip = getenv('HTTP_CLIENT_IP');
         }
         else
         {
            $realip = getenv('REMOTE_ADDR');
         }
      }

      preg_match("/[\d\.]{7,15}/", $realip, $onlineip);
      $realip = !empty($onlineip[0]) ? $onlineip[0] : '0.0.0.0';

      return $realip;
   }
}