WP - How to Flush Rewrite Rules for all sites on Multisite

flush_rewites_on_multisite.php

<?php
/**
 * Flush permalink on EVERY site on a mulsitite, so you don't have to visit
 * Permalink dashboard page of every single blog.
 *
 * How to use: 
 *    - upload it in /wp-content/mu-plugins/
 *    - leave the file there for some days 
 *    - then REMOVE the file!
 */
add_action('init', function() {

	if ( !is_multisite() ) {
		return;
	}

	$sites = wp_get_sites( array( 'limit' => false ) );

	foreach ( $sites as $site ) {

		// Flush sometimes, not on evey page load
		if ( get_transient( 'flushed_rules_'.$site['blog_id'] ) ) {
			continue;
		}

		switch_to_blog( $site['blog_id'] );

		flush_rewrite_rules( false );

		error_log( __FILE__ . '(' . __LINE__ . ') ' . __METHOD__ . '(): ' . 'flushed blod ID ' . $site['blog_id'] );

		set_transient( 'flushed_rules_'.$site['blog_id'], 'yes', 30 * DAY_IN_SECONDS );

		restore_current_blog();
	}
});