Consider the following scenario: You want to have several wikis on the same server and your users only want to login once. For this single-sign-on scenario to work you have to do the following things:

1. Change the definition of DOKU_COOKIE in inc/init.php to something that is the same on all instances, for example md5('commoncookie'). DOKU_COOKIE is the name of the cookie that is set after a user logs in. In a standard DokuWiki installation it's a hash of the full URL (including the path).
2. In the two setcookie calls in inc/auth.php, change DOKU_REL to '/'. This ensures that only one cookie is sent to all instances on the server.
3. The contents of the file data/meta/_htcookiesalt must be the same on all instances. The file gets created when a user first logs in, after that, the content stays the same.
4. To share the user information, declare one Wiki as the “master” wiki. The file conf/users.auth.php in the “slave” wikis must be a hard link to this file in the “master” wiki. If you have a similar namespace structure or don't need complex ACL setups, you can also share the file conf/acl.auth.php.

There are many other common login feasible like LDAP and Active Directory just search and install

The original instructions said in Step 4, “The file conf/users.auth.php in the “slave” wikis must be a symlink to this file in the “master” wiki.” When I tried using a symbolic link, I got the error messages,

! User authentication is temporarily unavailable. If this situation persists, please inform your Wiki Admin.
! No ACL setup yet! Denying access to everyone.

I made sure the link was not read-only, and that the permissions were the same as those on a working installation, in particular that the Internet Guest Account has Read and Write permissions. When I used a hard link, the set-up worked as advertised. I have changed the instructions accordingly, even though I don't understand what is going on. –Art Carlson, Max Planck Institute of Biochemistry, Munich, Germany