The following function will return the root domain given a URL.

get-root-domain.php

<?php
/*
	Get Root Domain from a URL with PHP
	http://www.internoetics.com/2015/05/04/get-root-domain-from-a-url-with-php/
*/

function internoetics_get_domain($url) {
  $pieces = parse_url($url);
  $domain = isset($pieces['host']) ? $pieces['host'] : '';
  if (preg_match('/(?P<domain>[a-z0-9][a-z0-9\-]{1,63}\.[a-z\.]{2,6})$/i', $domain, $m)) {
    return $m['domain'];
  }
 return false;
}

/* Usage: returns internoetics.com */
echo internoetics_get_domain("http://something.something.internoetics.com/directory/filename.php");
// returns internoetics.com
?>