Share users database from one Drupal database with other Drupal Installs (separate databases, not just prefixed tables in the same db)
Instead of using table prefixes, you can use a reference to another database as a prefix. For example, core database holds the shared users / permissions, drupal_website database is your actual website.



$databases['default']['default'] = array(
    'driver' =&gt; 'mysql',
    'database' =&gt; 'drupal_website',
    'username' =&gt; 'db_username',
    'password' =&gt; 'db_password,
    'host' =&gt; 'ip_db_server',
    'prefix' =&gt; array(
         'default' =&gt; '',
         'users' =&gt; 'core.',
         'users_roles' =&gt; 'core.',
         'sessions' =&gt; 'core.',
         'role' =&gt; 'core.',
         'role_permission' =&gt; 'core.',
         'authmap' =&gt; 'core.',
    ),
);


Minor drawback, your database needs to be stored on the same mysql server and each database uses the same credentials (username / password) for the connection.