gistfile1.txt

function get_site_id_of_user( $user_id, $all = false ) {
	global $wpdb;
	$user_id = (int) $user_id;
	// Logged out users can't have sites
	if ( empty( $user_id ) ) {
		return array();
	}

	$keys = get_user_meta( $user_id );
	if ( empty( $keys ) ) {
		return array();
	}
	
	$site_ids = array();
	if ( isset( $keys[ $wpdb->base_prefix . 'capabilities' ] ) && defined( 'MULTISITE' ) ) {
		$site_ids[1] = $keys[ $wpdb->base_prefix . 'capabilities' ];
		unset( $keys[ $wpdb->base_prefix . 'capabilities' ] );
	}

	foreach ( $keys as $key => $value) {
		if ( 'capabilities' !== substr( $key, -12 ) ) {
			continue;
		}
		if ( $wpdb->base_prefix && 0 !== strpos( $key, $wpdb->base_prefix ) ) {
			continue;
		}
		$site_id = str_replace( array( $wpdb->base_prefix, '_capabilities' ), '', $key );
		if ( ! is_numeric( $site_id ) ) {
			continue;
		}
		$site_ids[(int) $site_id] = $value;
	}
   return $site_ids;
  }