Themed Multiple Domains in Wordpress

Traditionally, wordpress does not allow you to use multiple domains on your site.  It will always redirect them to the URL you have specified in your options.  Generally speaking, that's exactly the right way to go about it unless you have an advanced purpose in mind.

Why would you want this?

Well, if you have multiple domains relating to your site (including subdomains) you can use this method to serve alternative content on those domains, from a single wordpress installation.  You can even use an entirely different theme for each domain, if you like.

Why did ∗I∗ want this?

Well, I am working on a project which needs to be accessible from a mobile phone or PDA.  A method using browser-detection is unreliable, simply switching out style sheets may result in elements and images being downloaded even if they're not displayed -- so I decided to create an entirely new (much lighter) wordpress theme and serve it from a .mobi domain.  After all, that's what .mobi domains are for, right?

Surely this is 'out there' already...

Well, yes, sort of.  When poking around for a ready-made solution I found a guy who thanked another guy for making a plugin to do this.  It's kind of old and not particularly well supported, so I wasn't really overjoyed at the thought of making it an essential part of my new project's future.

The other thing that bothered me is that the plugin was really quite active.  It filters all the generated links on your site, and sifts through all your content looking for absolute links to edit to match your additional domain.  This is probably exactly what you want if you have heaps of posts with hard links in them by the time you decide you want a new domain added, but I don't need that.

If that is what you want, there's a more up-to-date and quite nice-looking plugin available from this guy.

Why I didn't want a plugin

It's not that I don't like plugins, it's just that this is going to be core functionality for my new project, so I really don't want to have to wait for the plugin to be updated if there are major changes to the wordpress platform (quite common).  I want something simple, and I'm prepared to use relative links in my posts from day one.

How to do it

It's actually surprisingly simple.  You just need to modify your wp-config.php to set a few constants.

In wp-config.php find:

$table_prefix = 'wp_';

Underneath that, insert the new code:

function domain_theme($domain) {
   // edit this path to match your server
   $themepath = '/path/to/public_html/wp-content/themes/';
   define('WP_SITEURL', 'http://'.$domain);
   define('WP_HOME', 'http://'.$domain);
   define('TEMPLATEPATH', $themepath.$domain);
   define('STYLESHEETPATH', $themepath.$domain);
}
$domain = $_SERVER['SERVER_NAME'];
$domain = str_replace('www.', '', $domain);
// use one or more additional domains here
if ($domain == 'altsite.mobi' ||
    $domain == 'anothersite.net' ||
    $domain == 'thirdsite.com'
   ) domain_theme($domain);

You don't need to put your default domain in the last if-statement, this is just to verify that the domains you want to be themed are in fact set with their own theme folder which is named after the domain itself.

if you have lots and lots of domains with their own themes, you can change the last if-statement to read:

if ($domain !== 'defaultsite.com') domain_theme($domain);

That will result in errors unless you have a theme for each domain already uploaded.

If you want to allow multiple domains, but want to use the same theme for each domain, you can simply remove the lines containing TEMPLATEPATH and STYLESHEETPATH.

Drawbacks

If you don't have substantially different content on each of your domains Google will penalise you heavily.  You can avoid this by using a robots.txt file for your additional domains.  This is ideal for cases like mine, where you're only serving mobile content from the additional domain.

Preventing the Indexing of Duplicate Content

UPDATE: I've been asked to provide an example of how to block duplicate content using robots.txt files, so here's how I'm doing it:

I'm using two distinct robots.txt files.  One with the regular name, and another called robots_blocked.txt.  I then differentiate between them using .htaccess and mod_rewrite, information about which is infinitely googlable.

Here's what I have in my .htaccess file, directly above the wordpress section which should already be in there if you're using 'pretty' permalinks:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_HOST} !^(www\.)?yourmainsite\.com$ [NC]
RewriteRule ^robots.txt$ /robots_blocked.txt [NC,L]
</IfModule>

This basically ignores any requests for robots.txt via your main site, instead passing them to the real robots.txt file as they should be.  If you try to access robots.txt via any other domain (or subdomain) the rule is activated, and you're served robots_blocked.txt instead.

For the sake of the internet and your ranking in search engines, please ensure that any duplicate content (that is, the vast majority of your site) is properly excluded via your robots_blocked.txt