Question: In my host, I currently have installed 2 wordpress applications, 1 phpBB forum and one MediaWiki.

Is there a way to merge the login so that all applications share the same credentials?

For instance, I want to register only in my phpBB and then I want to access all other applications with the given username and password.

Even if you don't know a unified way, what other login integration do you know of? Pros and cons of each?

Answer: 
If you're integrating a bunch of different apps, and you really just want a bridge, I've had good success with the bridge from [Single-Signon.com](http://www.single-signon.com). You can see there supported apps here:
<http://www.single-signon.com/en/applications.html>

I've also used a MediaWiki extension for phpBB integration:
<http://www.mediawiki.org/wiki/Extension:PHPBB/Users_Integration>
Having tried to do this some years ago I remember it not being very easy. 

The way I did it was to create totally new table to user/pass and then replace these columns in the respective software with foreign keys to your new table - this required **a lot** of custom tweaking of core files in each application - mainly making sure all SQL requests to this data have the extra join needed for your new table. If I find the time I will maybe try and provide a step by step of the changes needed.

There are some **pretty big drawbacks** to this approach though. The main one being from now on your gonna have to hand update any patches 

If you have no content or users yet look at <http://bbpress.org/documentation/integration-with-wordpress/> which will make things a lot simpler for you.

I can't quite remember but I believe that I big problem I had was that MediaWiki requires usernames formatted a certain that conflicted with phpBB.

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Of course, a totally different approach would be to mod each piece of software to use OpenID \_ I believe plugins/extensions are readily available for all the applications you mentioned.