Multisite Network: How to change permalink structure programmatically on new blogs

Question

I have a subdomain multisite network. I have implemented a system in order to create new subdomains programmatically, just filling a form.

Now I have a need: when a new blog is created, I'd like to set its permalink structure to /postname/.

I've tried these solutions:

How to set permalink structure via functions.php

function set_default_permalink_for_new_blogs($blog_id) 
{
    global $wp_rewrite;
    $wp_rewrite->set_permalink_structure( '/%year%/%monthnum%/%postname%/' );
}
add_action('wpmu_activate_blog', 'set_default_permalink_for_new_blogs');

I've added this snippet in the functions.php of the theme used for the main site, where the system above mentioned is. It simply doesn't work.

How Do I Programmatically Force Custom Permalinks with My Theme?

function change_permalinks() {
    global $wp_rewrite;
    $wp_rewrite->set_permalink_structure('/%postname%/');
    $wp_rewrite->flush_rules();
}
add_action('init', 'change_permalinks');

I have some concerns with this snippet: I don't think it could be a good idea to fire the rewrite on every "init" (also someone else expressed doubt about this in comments); in second place, this snippet has to be placed on the default theme of new blogs - but what if I want to change the default theme in future?

Thank you very much for every suggestions,

Marco

Answer 1

With a "mu-plugins", the solution is...

add_action( 'wpmu_new_blog', function( $blog_id ){

switch_to_blog( $blog_id );
global $wp_rewrite;
$wp_rewrite->set_permalink_structure('/%postname%/');
$wp_rewrite->flush_rules();
restore_current_blog();

}, 10 );

Answer 2

You should be able to create an mu-plugin (required plugin) that fires on the activate_blog hook. Something like this should get you started in the right direction:

<?php
/* Plugin name: WPSE Permalink Setter
Description: Sets permalink structure for each blog activation
*/
add_action('activate_blog', 'wpse_271662_set_permalinks');
function wpse_271662_set_permalinks() {
    global $wp_rewrite;
    $wp_rewrite->set_permalink_structure('/%postname%/');
    $wp_rewrite->flush_rules();
}
?>

Since the code only runs when a new site is created (activate_blog), this prevents performance issues associated with running on every init, and it doesn't rely on a theme. Of course in theory, site admins could change the permalink structure later, but this would give them a clean install with the permalink structure you want, unless they monkey around with it.

Place this file directly inside /wp-content/mu-plugins/ (no subfolder like normal plugins) so it will run automatically unless the file is deleted.