consistency between $wpdb->blogid and get_current_blog_id()

[arena]

On a single install (not multisite),

you have

$wpdb->blogid => 0

get_current_blog_id() => 1

Must be a very old one !!!

[SergeyBiryukov]

Related: #19160, #41684.

[keraweb]

A check in the wpdb::__construct() for is_multisite() could fix this. If it's not a multisite then set blogid to 1.

Trac Ticket: Core-45761

## Problem Statement:

• In single-site WordPress installations, there is an inconsistency between the blogid property of the global $wpdb object and the value returned by get_current_blog_id(). Specifically:

$wpdb->blogid returns 0;
get_current_blog_id() returns 1;

This inconsistency is present in older installations and can lead to discrepancies in code that relies on the blogid value, especially in non-multisite environments.

## Fix:

• To resolve this issue, we have modified the wpdb::__construct constructor method. The fix includes:

• Adding an is_multisite() check within the constructor.

Setting $wpdb->blogid to 1 if is_multisite() returns false, ensuring consistency with the value returned by get_current_blog_id().