<?php
// get base domain (domain.tld)
/*____________________________________________________________________________*/
function get_base_domain($url) 
{
  $debug = 0;  
  $base_domain = '';
  
  // generic tlds (source: http://en.wikipedia.org/wiki/Generic_top-level_domain)
  $G_TLD = array(
    'biz','com','edu','gov','info','int','mil','name','net','org',
    'aero','asia','cat','coop','jobs','mobi','museum','pro','tel','travel',
    'arpa','root',
    'berlin','bzh','cym','gal','geo','kid','kids','lat','mail','nyc','post','sco','web','xxx',
    'nato',
    'example','invalid','localhost','test',
    'bitnet','csnet','ip','local','onion','uucp',
    'co'   // note: not technically, but used in things like co.uk
  );
  
  // country tlds (source: http://en.wikipedia.org/wiki/Country_code_top-level_domain)
  $C_TLD = array(
    // active
    'ac','ad','ae','af','ag','ai','al','am','an','ao','aq','ar','as','at','au','aw','ax','az',
    'ba','bb','bd','be','bf','bg','bh','bi','bj','bm','bn','bo','br','bs','bt','bw','by','bz',
    'ca','cc','cd','cf','cg','ch','ci','ck','cl','cm','cn','co','cr','cu','cv','cx','cy','cz',
    'de','dj','dk','dm','do','dz','ec','ee','eg','er','es','et','eu','fi','fj','fk','fm','fo',
    'fr','ga','gd','ge','gf','gg','gh','gi','gl','gm','gn','gp','gq','gr','gs','gt','gu','gw',
    'gy','hk','hm','hn','hr','ht','hu','id','ie','il','im','in','io','iq','ir','is','it','je',
    'jm','jo','jp','ke','kg','kh','ki','km','kn','kr','kw','ky','kz','la','lb','lc','li','lk',
    'lr','ls','lt','lu','lv','ly','ma','mc','md','mg','mh','mk','ml','mm','mn','mo','mp','mq',
    'mr','ms','mt','mu','mv','mw','mx','my','mz','na','nc','ne','nf','ng','ni','nl','no','np',
    'nr','nu','nz','om','pa','pe','pf','pg','ph','pk','pl','pn','pr','ps','pt','pw','py','qa',
    're','ro','ru','rw','sa','sb','sc','sd','se','sg','sh','si','sk','sl','sm','sn','sr','st',
    'sv','sy','sz','tc','td','tf','tg','th','tj','tk','tl','tm','tn','to','tr','tt','tv','tw',
    'tz','ua','ug','uk','us','uy','uz','va','vc','ve','vg','vi','vn','vu','wf','ws','ye','yu',
    'za','zm','zw',
    // inactive
    'eh','kp','me','rs','um','bv','gb','pm','sj','so','yt','su','tp','bu','cs','dd','zr'
    );
  
  
  // get domain
  if ( !$full_domain = get_url_domain($url) )
  {
    return $base_domain;
  }
  
  // now the fun
  
    // break up domain, reverse
    $DOMAIN = explode('.', $full_domain);
    if ( $debug ) print_r($DOMAIN);
    $DOMAIN = array_reverse($DOMAIN);
    if ( $debug ) print_r($DOMAIN);
    
    // first check for ip address
    if ( count($DOMAIN) == 4 && is_numeric($DOMAIN[0]) && is_numeric($DOMAIN[3]) )
    {
      return $full_domain;
    }
    
    // if only 2 domain parts, that must be our domain
    if ( count($DOMAIN) <= 2 ) return $full_domain;
   
    if ( in_array($DOMAIN[0], $C_TLD) && in_array($DOMAIN[1], $G_TLD) && $DOMAIN[2] != 'www' )
    {
      $full_domain = $DOMAIN[2] . '.' . $DOMAIN[1] . '.' . $DOMAIN[0];
    }
    else
    {
      $full_domain = $DOMAIN[1] . '.' . $DOMAIN[0];;
    }
  
  // did we succeed?  
  return $full_domain;
} 
/*____________________________________________________________________________*/


// get domain from url
/*____________________________________________________________________________*/
function get_url_domain($url) 
{
  $domain = '';
  
  $_URL = parse_url($url);
  
  // sanity check
  if ( empty($_URL) || empty($_URL['host']) )
  {
    $domain = '';
  }
  else
  {
    $domain = $_URL['host'];
  }
  
  return $domain;
} 
/*____________________________________________________________________________*/


// Testbed
/*____________________________________________________________________________*/

if ( 1 )
{
 // test code here
  $TESTURL[] = 'http://www.ahoy.kahoy.smp.com';   
  foreach ( $TESTURL as $url )
  {
    echo $url . ' -> ' . get_base_domain($url) . '
';
  }
}
?>