WordPress Multisite – global categories

Setting up a WP multisite instance – the client has an existing ontology / set of categories that they want to classify all content across the set of blogs. Also the desire is that any new categories would be added at the ‘network blog’ level and synced to the other blogs.

What’s the best way of doing this?


function __add_global_categories( $term_id )
    if ( get_current_blog_id() !== BLOG_ID_CURRENT_SITE || ( !$term = get_term( $term_id, 'category' ) ) )
        return $term_id; // bail

    if ( !$term->parent || ( !$parent = get_term( $term->parent, 'category' ) ) )
        $parent = null;

    global $wpdb;

    $blogs = $wpdb->get_col( "SELECT blog_id FROM {$wpdb->blogs} WHERE site_id = '{$wpdb->siteid}'" );
    foreach ( $blogs as $blog ) {
        $wpdb->set_blog_id( $blog );

        if ( $parent && ( $_parent = get_term_by( 'slug', $parent->slug, 'category' ) ) )
            $_parent_ID = $_parent->term_id;
            $_parent_ID = 0;

        wp_insert_term( $term->name, 'category',  array(
            'slug' => $term->slug,
            'parent' => $_parent_ID,
            'description' => $term->description

    $wpdb->set_blog_id( BLOG_ID_CURRENT_SITE );
add_action( 'created_category', '__add_global_categories' );

This will run whenever a category is added on the main site. A few caveats/points worth mentioning;

  • If you have a lot of blogs, this function may get pretty intensive.
  • On average, we are running anywhere between 5 to 8 queries (possibly more) per blog – depending on the speed of your database, this function may need to be chunked.
  • Only newly added categories are ‘synced’. Updating and deleting categories are not (code will need to be revised).
  • If a newly added category has a parent, and the parent cannot be found within the multisite blog in question, the category will be created with no parent (this should only be the case if the parent category was created before this function was installed).

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