When you use the WP_CACHE constant in the wp-config.php file, it activate the cache for WordPress. This features reduce the server load and improves the site’s speed for a smoother experience for users.
In versions of WordPress before 2.5, if you included define(‘WP_CACHE’, true) in the wp-config.php file, the data stored through wp_cache functions would stick around even after you closed the website. But things have changed now. Simply adding the define won’t do anything unless you’ve installed a special plugin for persistent caching.
If you don’t specifically define it, WordPress will automatically set WP_CACHE to false, and it won’t load the advanced-cache.php file by default.
Caching plugins often mention the WP_CACHE constant in their setup files. When multiple caching methods are active simultaneously, conflicts can arise. To see if the issue notice disappears, try temporarily deactivating caching plugins.