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copied!<p><strong><em>Note: as pointed out in the comments, this method doesn't actually work in all cases. The reason for this is that some top-level domains do resolve to IP addresses, even if most do not. Therefore it's not possible to detect if a given name is top-level or pseudo-top-level domain name merely by checking if it has an IP address. Unfortunately, this probably means that the only solution is a lookup list, given how inconsistently treated top-level domains are in practice.</em></strong></p> <p><strong><em>I repeat: do not rely on the code below to work for you. I leave it here for educational purposes only.</em></strong></p> <p>There is a way to do this without a lookup list. The list may be unreliable or incomplete, whereas this method is guaranteed to work:</p> <pre><code><?php function get_domain($url) { $dots = substr_count($url, '.'); $domain = ''; for ($end_pieces = $dots; $end_pieces > 0; $end_pieces--) { $test_domain = end(explode('.', $url, $end_pieces)); if (dns_check_record($test_domain, 'A')) { $domain = $test_domain; break; } } return $domain; } $my_domain = get_domain('www.robknight.org.uk'); echo $my_domain; ?> </code></pre> <p>In this case, it will output 'robknight.org.uk'. It would work equally well for .com, .edu, .com.au, .ly or whatever other top-level domain you're operating on.</p> <p>It works by starting from the right and doing a DNS check on the first thing that looks like it might be a viable domain name. In the example above, it starts with 'org.uk', but discovers that this is not an actual domain name, but is a ccTLD. It then moves on to check 'robknight.org.uk', which <em>is</em> valid, and returns that. If the domain name had been, say, 'www.php.net', it would have started by checking 'php.net', which is a valid domain name, and would have returned that immediately without looping. I should also point out that if no valid domain name is found, an empty string ('') will be returned.</p> <p>This code may be unsuitable for processing a large number of domain names in a short space of time due to the time taken for DNS lookups, but it's perfectly fine for single lookups or code that isn't time-critical.</p>

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