PHP Tricks: How To Handle Multiple Domains

This is really handy for those of us who have the same code handling multiple sites or multiple sub-domains.

A case in point: When I coded NetBoardz (my free forum hosting service now defunct), I had one codebase handling all 250 forums. How? Simple. When the code runs, it determines which site the user is loading and does different things (like using different databases) dynamically.

How to determine the domain the user is using to view your site:

$domain = $_SERVER['HTTP_HOST'];
if ($domain == "xyz") {
    ...
} else if ($domain == "uvw") {
    ...
}

In the example above you can see that we have put the domain that the user has used to view your site into the $domain variable, loading the value from the PHP global variable, $_SERVER. The $_SERVER variable is global, which means you can access it anytime and anywhere in your code.

More information on PHP’s predefined global variables.

How to determine the sub-domain the user is using to view your site:

Sample code is from NetBoardz, which is based off of phpBB 2:

$subdomain = strpos($_SERVER['HTTP_HOST'], ".");
$subdomain = substr($_SERVER['HTTP_HOST'], 0, $subdomain);
$dbname = "nb_".$subdomain;
mysql_select_db($dbname, $sql_link);

Here you can see that we retreived the whole hostname, including the top-level domain and subdomain, then used the PHP functions strpos and substr to take anything before the first dot. For example, the whole hostname “testforum.netboardz.com” passed through this code would end up as “testforum”.

After, we use that subdomain name to calculate which forum database to load. Of course, once you have the domain or subdomain in a variable, you are able to handle your code as you wish!

Validating Domain Names and Websites

If you’re ever in need of a regular expression that will validate every domain name to make sure what the user gave you is valid, you can use Shaun Inman’s Regex. I have to admit, while it’s pretty uber, it’s probably easier just to use cURL to see if the URL is valid.

To accomplish website validation using cURL, there is a snippet of code available on the official PHP homepage: http://ca3.php.net/manual/en/function.curl-exec.php#77167. If the HTTP status is 200 after requesting the website using cURL, the website is valid. If not, you can return an error to the user to verify their URL.