获得用户的真实IP地址

/**
* 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;

}