wp_validate_logged_in_cookie

wp_validate_logged_in_cookie() comment from source:

/**
 * Validates the logged-in cookie.
 *
 * Checks the logged-in cookie if the previous auth cookie could not be
 * validated and parsed.
 *
 * This is a callback for the {@see 'determine_current_user'} filter, rather than API.
 *
 * @since 3.9.0
 *
 * @param int|bool $user_id The user ID (or false) as received from the
 *                       determine_current_user filter.
 * @return int|false User ID if validated, false otherwise. If a user ID from
 *                   an earlier filter callback is received, that value is returned.
 */

It is a default callback function for the filter hook determine_current_user used by the private function _wp_get_current_user() and some plugins.

Its default usage as defined in wp-includes/default-filters.php line 371:

add_filter( 'determine_current_user', 'wp_validate_logged_in_cookie', 20 );

_wp_get_current_user()‘s use of the filter from line 2523 of wp-includes/user.php:

     /**
     * Filters the current user.
     *
     * The default filters use this to determine the current user from the
     * request's cookies, if available.
     *
     * Returning a value of false will effectively short-circuit setting
     * the current user.
     *
     * @since 3.9.0
     *
     * @param int|bool $user_id User ID if one has been determined, false otherwise.
     */
    $user_id = apply_filters( 'determine_current_user', false );

In this usage wp_validate_logged_in_cookie() is passed the argument false, and would thus be forced to run wp_validate_auth_cookie() if there is a cookie and we are on the frontend.

wp_validate_auth_cookie() returns a user id if the cookie is valid.

---

The full source of _wp_get_current_user(), to see the above quoted apply_filters() line in context. Note that if no user id is returned to it, user id is set to an invalid 0 and immediately returned; denoting no logged in user.

function _wp_get_current_user() {
    global $current_user;

    if ( ! empty( $current_user ) ) {
        if ( $current_user instanceof WP_User ) {
            return $current_user;
        }

        // Upgrade stdClass to WP_User
        if ( is_object( $current_user ) && isset( $current_user->ID ) ) {
            $cur_id = $current_user->ID;
            $current_user = null;
            wp_set_current_user( $cur_id );
            return $current_user;
        }

        // $current_user has a junk value. Force to WP_User with ID 0.
        $current_user = null;
        wp_set_current_user( 0 );
        return $current_user;
    }

    if ( defined('XMLRPC_REQUEST') && XMLRPC_REQUEST ) {
        wp_set_current_user( 0 );
        return $current_user;
    }

    /**
     * Filters the current user.
     *
     * The default filters use this to determine the current user from the
     * request's cookies, if available.
     *
     * Returning a value of false will effectively short-circuit setting
     * the current user.
     *
     * @since 3.9.0
     *
     * @param int|bool $user_id User ID if one has been determined, false otherwise.
     */
    $user_id = apply_filters( 'determine_current_user', false );
    if ( ! $user_id ) {
        wp_set_current_user( 0 );
        return $current_user;
    }

    wp_set_current_user( $user_id );

    return $current_user;
}