The domain is stored in $_SERVER[ HTTP_HOST ].

EDIT: I believe this returns the whole domain. To just get the top-level domain, you could do this:

// Add all your wanted subdomains that act as top-level domains, here (e.g.  co.cc  or  co.uk )
// As array key, use the last part ( cc  and  uk  in the above examples) and the first part as sub-array elements for that key
$allowed_subdomains = array(
     cc     => array(
         co 
    ),
     uk     => array(
         co 
    )
);

$domain = $_SERVER[ HTTP_HOST ];
$parts = explode( . , $domain);
$top_level = array_pop($parts);

// Take care of allowed subdomains
if (isset($allowed_subdomains[$top_level]))
{
    if (in_array(end($parts), $allowed_subdomains[$top_level]))
        $top_level = array_pop($parts). . .$top_level;
}

$top_level = array_pop($parts). . .$top_level;

You can use parse_url() to split it up and get what you want. Here s an example...

    $url =  http://www.google.com/search?hl=en&source=hp&q=google&btnG=Google+Search&meta=lr%3D&aq=&oq=dasd ;
    print_r(parse_url($url));

Will echo...

Array
(
    [scheme] => http
    [host] => www.google.com
    [path] => /search
    [query] => hl=en&source=hp&q=google&btnG=Google+Search&meta=lr%3D&aq=&oq=dasd
)

I reckon you ll need a list of all suffixes used after a domain name. http://publicsuffix.org/list/ provides an up-to-date (or so they claim) of all suffixes in use currently. The list is actually here Now the idea would be for you to parse up that list into a structure, with different levels split by the dot, starting by the end levels:

so for instance for the domains: com.la com.tr com.lc

you d end up with:

[la]=>[com]
[lc]=>[com]

etc...

Then you d get the host from base_url (by using parse_url), and you d explode it by dots. and you start matching up the values against your structure, starting with the last one:

so for google.com.tr you d start by matching tr, then com, then you won t find a match once you get to google, which is what you want...

Regex and parse_url() aren t solution for you.

You need package that uses Public Suffix List, only in this way you can correctly extract domains with two-, third-level TLDs (co.uk, a.bg, b.bg, etc.). I recomend use TLD Extract.

Here example of code:

$extract = new LayerShifterTLDExtractExtract();

$result = $extract->parse( http://subsub.sub.google.com.tr );
$result->getRegistrableDomain(); // will return (string)  google.com.tr