Numerical ID of the current WordPress user is stored in this global variable. It is set when the current user is created in the pluggable functions
get_currentuserinfo() defined in [wordpress root folder]/wp-includes/pluggable-functions.php.
This global variable can be used to pass as a parameter to lot of WordPress functions which require this information. It is also useful when you have to build a custom SQL query for requesting information for this user. WordPress caches the user information by the user ID, it can be retrieved using this global variable, e.g.,
$user = wp_cache_get($user_ID, 'users');
Back to full list of global variables.
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