Finding the root domain name (and TLD, or top level domain) from a URL is challenging. Literature on the web generally leans towards a library that'll find the root domain by way of comparing the URL against all approved top level domains 
... but this method introduces a degree of complexity that we wanted to avoid. The following function is one that we use to find a domain name root (or TLD). It isn't perfect, but it's close.
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<?php
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/*
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Get Root Domain (or TLD) from a URL with PHP
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http://www.beliefmedia.com/root-domain-url-php
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*/
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$domain = isset($pieces['host']) ? $pieces['host'] : '';
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return ($tld === true) ? substr($m['domain'], ($pos = strpos($m['domain'], '.')) !== false ? $pos + 1 : 0) : $m['domain'];
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}
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return false;
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}
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/* Usage: returns 'beliefmedia.com' */
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echo beliefmedia_get_domain("http://something.something.beliefmedia.com/directory/filename.php");
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/* Usage: returns 'com.au' */
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// echo beliefmedia_get_domain("http://something.something.beliefmedia.com.au/directory/filename.php", $tld = true);
You may optionally return the top level domain (eg, .com.au or .net) with the argument $tld = true.
