HOWTO: Use a special template for certain categories

I wanted to use a different template for certain categories of posts, so I renamed single.php to single-default.php and created a new single.php to test the categories. If it is one of the special categories, use single-in-category.php, otherwise, use single-default.php.

I originally hard-coded the categories in the new single.php, but as I added new ones, I had to edit the code. My solution was to use https://planetozh.com/blog/my-projects/wordpress-theme-toolkit-admin-menu/.

This toolkit was easy to integrate with my theme; all I had to do was copy themetoolkit.php to my theme directory and add one function to functions.php. Then it was simple to add an option called single_in_category. This contains a comma separated list of the category slugs or IDs. Now I can add or remove a category in the Admin page.

Here is my new single.php:

<?php
// single.php is now in single-default.php
// We want to use single-in-category for
// all categories listed in the single_in_category
// option which is set in the Appearance->Miata Club Options panel.
// Uses global $mclub created by themetoolkit in functions.php
global $mclub;
$post = $wp_query->post;
$mycats = array();
if ($mycatlist = $mclub->option['single_in_category']) {
  $mycats = explode(',',$mycatlist);
}

if ( in_category($mycats)) {
  include(TEMPLATEPATH . '/single-in-category.php'); }
else {
  include(TEMPLATEPATH . '/single-default.php');
}
?>