Source

File: wp-includes/class-wp-site.php

public function __set( $key, $value ) {
		switch ( $key ) {
			case 'id':
				$this->blog_id = (string) $value;
				break;
			case 'network_id':
				$this->site_id = (string) $value;
				break;
			default:
				$this->$key = $value;
		}
	}