<?php
$S = 'http://bbs.ruanchen.com';
$S = parse_url($S);
$S = strtolower($S['host']) ; //取域名部分
//echo $S;

$domain = array('com','cn','name','org','net'); //域名后缀 有新的就扩展这吧
$SS = $S;
$dd = implode('|',$domain);
$SS = preg_replace('/(\.('.$dd.'))*\.('.$dd.')$/iU','',$SS); 把后面的域名后缀部分去掉
 
$SS = explode('.',$SS);
$SS = array_pop($SS);  //取最后的主域名
$SS = substr($S,strrpos($S,$SS));  //加上后缀拼成完成的主域名
echo $SS;
?>