WP_User::get_site_id()
Gets the ID of the site for which the user’s capabilities are currently initialized.
Return
(int) Site ID.
Source
File: wp-includes/class-wp-user.php
public function get_site_id() {
		return $this->site_id;
	}  Changelog
| Version | Description | 
|---|---|
| 4.9.0 | Introduced. | 
    © 2003–2021 WordPress Foundation
Licensed under the GNU GPLv2+ License.
    https://developer.wordpress.org/reference/classes/wp_user/get_site_id