wp-includes/class-wp-network.php:155Getter.
$keystringmixed public function __get( $key ) { switch ( $key ) { case 'id': return (int) $this->id; case 'blog_id': return (string) $this->get_main_site_id(); case 'site_id': return $this->get_main_site_id(); } return null; }Introduced in 4.6.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-network.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.