/**
 * 获得用户的真实 IP 地址
 *
 * @return NULL Ambigous unknown>
 */
function get_client_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?个非unknown的有效IP字符? */
            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;
}