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: 
One option is OpenID, which you can integrate into [phpBB](http://sourceforge.net/projects/phpbb-openid/), [WordPress](http://wordpress.org/extend/plugins/openid/), and [MediaWiki](http://www.mediawiki.org/wiki/Extension:OpenID).

A second option is to set up an LDAP server, which you can also integrate into [phpBB](http://sourceforge.net/projects/ldapauthmod/), [WordPress](http://wordpress.org/extend/plugins/wpldap/), and [MediaWiki](http://www.mediawiki.org/wiki/Extension:LDAP_Authentication).

If the sites are all on the same root domain, a third option is to modify the registration, login, and logout code so that these actions are replicated on every site at the same time. This gets messy, but it may be the easiest short-term solution if you're in a hurry. Once you track down the account code in each site, it's just a matter of copying and pasting and changing a few cookie parameters.
I once did a phpBB/MediaWiki login integration from the phpBB end.

[Check it out](https://damnian.svn.sourceforge.net/svnroot/damnian/phpBB3/MediaWiki/).