PHP gets user's real IP address

/**
 * 获得用户的真实IP地址
 *
 * @access  public
 * @return  string
 */
function 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;
}