WordPress at a glance
filter is not described

pre_get_main_site_id filter-hook . WP 4.9.0

Filters the main site ID.

Returning a positive integer will effectively short-circuit the function.

Usage

add_filter( 'pre_get_main_site_id', 'filter_function_name_7901', 10, 2 );
function filter_function_name_7901( $main_site_id, $network ){
	// filter...

	return $main_site_id;
}
$main_site_id(int/null)
If a positive integer is returned, it is interpreted as the main site ID.
$network(WP_Network)
The network object for which the main site was detected.

Where the hook is called

WP_Network::get_main_site_id()
wp-includes/class-wp-network.php 225
$main_site_id = (int) apply_filters( 'pre_get_main_site_id', null, $this );

Where the hook is used (in WP core)

Does not used.