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: 
I don't know how to share the session cookies, but you can easily share the same login. 
i.e. People will need to log separately into both sites, but will be able to use the same username and password. 

In the mediawiki file "LocalSettings.PHP", you can tell it to use a different (wordpress) database for authentication: 

e.g. 

```
require_once('includes/AuthPlugin.php');
require_once('extensions/AuthPress.php');

$wgAuth = new AuthPress();
$wgAuth->setAuthPressTablePrefix('evo_');
# Only include the following if you aren't using the same db as  MediaWiki
$wgAuth->setAuthPressDBServer ('localhost');
$wgAuth->setAuthPressDBName('yourWordPressDB');
$wgAuth->setAuthPressUser('mySQL user for same');
$wgAuth->setAuthPressPassword('The password');

```

See <http://bbpress.org/forums/topic/mediawiki-bbpress-and-wordpress-integration>
You can write a custom login hook for mediaWiki. I've done it for LibraryThing so that login credentials from our main site are carried over to our mediaWiki installation. The authentication hook extends mediaWiki's AuthPlugin.

There are several small issues: 

1. mediaWiki usernames must start with initial caps (so if you allow case sensitive user names it could be a problem if two users have colliding wiki names)
2. underscores in usernames are converted to spaces in mediaWiki

But if you can deal with those then it is certainly possible to use your own user/password data with mediaWiki.

Advantages: 

1. The user doesn't have to login to each area separately. Once they login to the main site they are logged into the wiki also.
2. You know that usernames are the same across the systems and can leverage that in links, etc.