function rLastParse($str, $sep, $cnt = 1, $complete_if_not = false)
{
 $i = strrpos($str, $sep);
 while (($cnt > 1) && ($i !== false)) {
  $i = strrpos(substr($str, 0, $i), $sep);
  $cnt--;
 }
 if ($i === false) {
  return $complete_if_not ? $str : ‘‘;
 }
 else {
  return substr($str, $i + strlen($sep));
 }
}

function getTopLevelDomain()
{
 if(isset($_SERVER[‘HTTP_X_FORWARDED_HOST‘]) && !empty($_SERVER[‘HTTP_X_FORWARDED_HOST‘])){
  $host = $_SERVER[‘HTTP_X_FORWARDED_HOST‘];
 }
 else{
  $host = $_SERVER[‘HTTP_HOST‘];
 }
 return rLastParse($host, DOT);
}

get top level domain by php