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 “” 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!

6 thoughts on “PHP Tricks: How To Handle Multiple Domains

  1. hi, nice article.. but i have question for you. How you include the code base to your subdomain that user has called? in my thougt you only have one code based that can be accesed from many subdomain, right? can you tell me how to require the base code in many subdomain so user can view the based code which the content is already called based on its subdomain?


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