There are many reasons why you’d want to share users on multiple WordPress sites. WordPress Networks is a great solution, but unfortunately isn’t for everyone – especially if you’re converting an existing site into a network. The below guide offers an alternative to sharing users, and works for WordPress 4.1.

There are going to be two states for your sites: a Master site which stores all of the user data; and subsequent Slave sites, which pull the user data from the Master’s table. Decide now which will be your Master site.

1.

Ensure both your Master and Slave sites are running on the same version of WordPress, and that they share a database. Obviously if they’re sharing a database, they need to use different table prefixes. For example, our parent is wpparent_ and our slave is wpslave_

From here on, we’ll only be editing the Slave site.

2.

Find the wp-config.php file on your Slave site (this should be in your WordPress root directory) and search for:

/* That's all, stop editing! Happy blogging. */

Just above this line, add the following:

define('CUSTOM_USER_TABLE', 'wp_users');
define('CUSTOM_USER_META_TABLE', 'wp_usermeta');

Make sure you replace the wp_ prefix with your parent prefix; so in this case we’d use wpparent_

Your two sites are now sharing the same user table! If you want to share users, but have different permissions for each site, then you can stop here. You’ll need to manually update a users permissions, as if they try and login to your Slave site, they’ll find that they don’t have sufficient permissions. This is because WordPress stores user permissions in another table.

3.

To copy over a users permissions as well, open up the wp_config.php file for your Slave site again and add:

define('CUSTOM_CAPABILITIES_PREFIX', 'wp_');

Underneath the lines you added previously, but above "That's all, stop editing! Happy blogging.". Again, make sure you change the WordPress prefix to your parent’s prefix.

4.

NOTE: As of WordPress 4.7.2 you will need to make the changes twice in wp-includes/class-wp-user.php. Just search for the code below and replace both areas where it appears. If you’re still running a version of WordPress pre-4.7.2, you only need to make the change once in wp-includes/capabilities.php.

Find the following code in either wp-includes/class-wp-user.php or wp-includes/capabilities.php:

$this->cap_key = $wpdb->get_blog_prefix() . 'capabilities';

Simply replace this line with:

if (defined ('CUSTOM_CAPABILITIES_PREFIX')) {
  $this->cap_key = CUSTOM_CAPABILITIES_PREFIX . 'capabilities';
} else {	
  $this->cap_key = $wpdb->get_blog_prefix() . 'capabilities';
}

And you should be golden!

It’s worth remembering that as we’re modifying core WordPress files, you may have to repeat these steps when you update your WordPress.