1차 도메인 정보만 추출하기 

function getBaseDomain($dom) { 
    $matches = array(); 
    preg_match('/[^\.]+\.([^\.]{4}|[^\.]{3}|(co|or|pe|ne|re|go|hs|ms|es|kg|sc|ac)\.[^\.]{2}|[^\.]{2})$/i', $dom, $matches); 
    return $matches[0]; 
} 

$domArr = array(); 
$domArr[] = 'a.b.c.d.domain1.co.kr'; 
$domArr[] = 'a.b.c.d.domain2.kr'; 
$domArr[] = 'a.b.c.d.domain3.com'; 
$domArr[] = 'a.b.c.d.domain4.net'; 
$domArr[] = 'a.b.c.d.domain5.bizy'; 
$domArr[] = 'a.b.c.d.domain6.org'; 
$domArr[] = 'a.b.c.d.domain7.info'; 
$domArr[] = 'a.b.c.d.domain8.pe.kr'; 
$domArr[] = 'naver.com'; 
$domArr[] = 'aaa.naver.com'; 
$domArr[] = 'aa.pe.cc'; 
$domArr[] = 'ab.co.jp'; 
$domArr[] = '234AS.asdfasf.co.jp'; 
$domArr[] = 'sir.kr'; 
$domArr[] = 'asdf.co.kr'; 
$domArr[] = 'www.asdf.co.kr'; 
$domArr[] = 'www.asdf.jp'; 
$domArr[] = 'www.asdf.ddd.ddd'; 

$len = sizeof($domArr); 
for($i=0 ;$i<$len; $i++) { 
    echo getBaseDomain($domArr[$i]) , "<br>".PHP_EOL; 
}