Use parse_url like below:
<?php
$url = 'http://www.example.com/news?q=string&f=true&id=1233&sort=true';
$values = parse_url($url);
$host = explode('.',$values['host']);
echo $host[1];
?>
This would work for any url that has the sub domain included (www. etc)
The PHP documentation can be found here: http://php.net/manual/en/function.parse-url.php
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