?php
/**
 * @Author: Ding Jianlong
 * @Date:  2019-03-07 16:14:04
 * @Last Modified by:  Ding Jianlong
 * @Last Modified time: 2019-03-20 13:45:12
 */
header('content-type:text/html;charset=utf-8');
//获取顶级域名
function getTopHost($url){
 $url = strtolower($url);  //首先转成小写
 $hosts = parse_url($url);
 $host = $hosts['host'];
 //查看是几级域名
  $data = explode('.', $host);
  $n = count($data);
  //判断是否是双后缀
  $preg = '/[\w].+\.(com|net|org|gov|edu)\.cn$/';
  if(($n > 2)  preg_match($preg,$host)){
   //双后缀取后3位
   $host = $data[$n-3].'.'.$data[$n-2].'.'.$data[$n-1];
  }else{
   //非双后缀取后两位
   $host = $data[$n-2].'.'.$data[$n-1];
  }
  return $host;
}
// 测试
echo getTopHost("http://ABC.com/s/j?wd=djl"),'br>';
echo getTopHost("http://www.abc.com/s/j?wd=djl"),'br>';
echo getTopHost("http://2.www.abc.com/s/j?wd=djl"),'br>';
echo getTopHost("https://mp.weixin.qq.com/s?__biz=MzA3ODI3ODUzMw=="),'br>';
echo getTopHost("http://cfi.net.cn/"),'br>';
echo getTopHost("http://www.cfi.NEt.cn/"),'br>';
echo getTopHost("https://www.sina.com.cn/?from=kandian"),'br>';