Fix the notice “COOKIE_DOMAIN constant already defined. The SSO and cart cross-domain features possible not fully functional.”
In WordPress the COOKIE_DOMAIN constant is used to specify the CMS cookie for unusual domain setups. The plugin use this constant to handle SSO and some of the cross-domain features.
If the constant is already defined, this must be removed or commented out. Most of the time it can be found in wp-config.php file and looks like this:
define( 'COOKIE_DOMAIN', 'mydomain.com' );
The notice appear on SuperAdmin dashboard on top of page:
In majority of cases the constant is defined by “WordPress MU Domain Mapping” plugin, which is used by few users to set-up different domains for sites in the network. This is however useless, the WooCommerce Global Cart take care of that functionality through WordPress core functions and additional classes. The above plugin should be de-activated and uninstalled.
If the notice still persist, it must be defined on another place or by another plugin, doing a server files search for ‘COOKIE_DOMAIN’ should bring up the exact spots where this is being used.