php 获取顶级域名

/**
     * 获取顶级域名
     * @param $url
     * @return string
     */
    public static function getDoMain($url){
        if(empty($url)){
            return '';
        }
        if(strpos($url,'http://') !== false){
            $url = str_replace('http://','',$url);
        }
        if(strpos($url,'https://') !== false){
            $url = str_replace('https://','',$url);
        }
        $n = 0;
        for($i = 1;$i <= 3;$i++) {
            $n = strpos($url, '/', $n);
            $i != 3 && $n++;
        }

        $nn = strpos($url, '?');
        $mix_num =  min($n,$nn);
        if($mix_num > 0 || !empty($mix_num)){
            //防止链接带有点 （.） 导致出错
            $url = mb_substr($url,0,$mix_num);
        }
        $data = explode('.', $url);

        $co_ta = count($data);
        //判断是否是双后缀
        $no_tow = true;
        $host_cn = 'com.cn,net.cn,org.cn,gov.cn';
        $host_cn = explode(',', $host_cn);
        foreach($host_cn as $val){
            if(strpos($url,$val)){
                $no_tow = false;
            }
        }
        //截取域名后的目录
        $del = strpos($data[$co_ta-1], '/');
        if($del > 0 || !empty($del)){
            $data[$co_ta-1] = mb_substr($data[$co_ta-1],0,$del);
        }
        //如果是返回FALSE ，如果不是返回true
        if($no_tow == true){
            $host = $data[$co_ta-2].'.'.$data[$co_ta-1];
        }else{
            $host = $data[$co_ta-3].'.'.$data[$co_ta-2].'.'.$data[$co_ta-1];
        }

        return $host;
    }