php 获取顶级域名、一级域名

/**
 * 获取顶级域名
 * @return [type] 
 * 比如reserve.applinzi.com返回applinzi.com
 */
function get_host($to_virify_url = ''){
    
    $url   = $to_virify_url ? $to_virify_url : $_SERVER['HTTP_HOST'];
    $data = explode('.', $url);
    $co_ta = count($data);

    //判断是否是双后缀
    $zi_tow = true;
    $host_cn = 'com.cn,net.cn,org.cn,gov.cn';
    $host_cn = explode(',', $host_cn);
    foreach($host_cn as $host){
        if(strpos($url,$host)){
            $zi_tow = false;
        }
    }

    //如果是返回FALSE ，如果不是返回true
    if($zi_tow == true){

        // 是否为当前域名
        if($url == 'localhost'){
            $host = $data[$co_ta-1];
        }
        else{
            $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;
}