Source

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

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