Filter by Multisite

filter-multisite.php

<?php
/**
 * Add multisite filter support for search.
 *
 * @param array $query
 *
 * @since 1.0.0
 */

function filter_multisite( $query ) {
	if ( ! is_admin() && $query->is_main_query() && $query->is_search() ) {
		$filter_multisite = ! empty( $_GET[ 'filter_multisite' ] ) ? (int) $_GET[ 'filter_multisite' ] : '';
		$sites = get_sites();
		$existing_sites = array();

		foreach ($sites as $site) {
			$existing_sites[] = (int) $site->blog_id;
		}

		if ( ! empty( $filter_multisite ) && in_array( $filter_multisite, $existing_sites ) ) {
			switch_to_blog( $filter_multisite );
		}
	}

	return $query;
}

add_filter( 'pre_get_posts', 'filter_multisite' );