PHP get primary domain

get_domain.php

<?php

function get_domain($host){
  $myhost = strtolower(trim($host));
  $count = substr_count($myhost, '.');

  // www.(domain.com) || (domain.co.uk) || sub.(domain.com)
  if($count === 2){
    if(strlen(explode('.', $myhost)[1]) > 3) $myhost = explode('.', $myhost, 2)[1];

  // re-run the function after removing first subdomain segment | www.sub.(domain.com) or www.(domain.co.uk) or www.sub.sub.sub.(domain.co.uk)
  } else if($count > 2){
    $myhost = get_domain(explode('.', $myhost, 2)[1]);
  }

  return $myhost;
}

?>