Question

I would appreciate any help that can be provided with this matter.

I am creating a registration form, one field is for the users domain which I will verify is valid with FILTER_VALIDATE_URL and that it exists with dns_check_record.

However a problem I’m having is that using these two methods will also allow subdomains to be submitted to the form which I don’t want.

Does anyone know a way to allow domains but not subdomains?

I’ve tested the following function, from http://syntax.cwarn23.net/PHP/Strip_URL_to_Domain:

function domain($domainb)
{
$bits = explode('/', $domainb);
if ($bits[0]=='http:' || $bits[0]=='https:')
    {
    $domainb= $bits[2];
    } else {
    $domainb= $bits[0];
    }
unset($bits);
$bits = explode('.', $domainb);
$idz=count($bits);
$idz-=3;
if (strlen($bits[($idz+2)])==2) {
$url=$bits[$idz].'.'.$bits[($idz+1)].'.'.$bits[($idz+2)];
} else if (strlen($bits[($idz+2)])==0) {
$url=$bits[($idz)].'.'.$bits[($idz+1)];
} else {
$url=$bits[($idz+1)].'.'.$bits[($idz+2)];
}
return $url;

However this isn’t perfect as any domains such as http://www.domain.uk.com will appear as uk.com (I know not a common domain extension).

Does anyone know a method better than the above function?

Answer 1

As pointed by Micheal Mior, you have to check for .co.uk, .com.br and many others.

Some browser vendors are maintaining a list of such non-TLD that are effectively TLD: http://publicsuffix.org/. The list is quite huge.

There is a library here that uses this effective TLD list to implement the function you are looking for (download are here). (Found via https://wiki.mozilla.org/Gecko:Effective_TLD_Service.)