Specify multiple categories for custom post template - FATAL ERROR

Question

I am using this code below to specify a post template for a specific category in my functions.php file.

function get_custom_cat_template($single_template) {
 global $post;

   if ( in_category( 'ms-conversations' )) {
      $single_template = dirname( __FILE__ ) . '/CUSTOM-POST-BLOG-POST.php';

 }
 return $single_template;

}

add_filter( "single_template", "get_custom_cat_template" ) ; 

The code technically works, however I need to specify about 20 categories for that same post template.

When I copy and paste the code again and again I get this fatal error because I am declaring it twice, but I do not know how to include all of the categories I need.

Fatal error: Cannot redeclare get_custom_cat_template() (previously declared in /home/content/12/9195112/html/wp-hoff-testing/wp-content/themes/dw-focus/functions.php:152) in /home/content/12/9195112/html/wp-hoff-testing/wp-content/themes/dw-focus/functions.php on line 174

Here is a list of all the categories I need for that post template.

ms-conversations, artist-of-the-month, exercise, hiking-for-multiple-sclerosis, msaa, caregiving-msaa, updates, ms-resources, ms-tips-msaa, recipe-of-the-month, stories-to-inspire, well-being, guest-bloggers, ms-publications, videos, multiplesclerosis-net, sharkfest, surveys, swim-for-ms, the-motivator

When I try to use commas or & signs to separate the categories I get further errors, how do I declare multiple categories for this chunk of code?

Thank you.

Answer 1

You don't need multiple functions. Use an array with in_category to pass multiple slugs:

if( in_category( array( 'ms-conversations', 'artist-of-the-month' ) ) ){
    // do something
}

Answer 2

This is my final code.

function get_custom_cat_template($single_template) {
 global $post;

   if( in_category( array( 'ms-conversations', 'artist-of-the-month', 'exercise', 'hiking-for-multiple-sclerosis', 'msaa', 'caregiving-msaa', 'updates', 'ms-resources', 'ms-tips-msaa', 'recipe-of-the-month', 'stories-to-inspire', 'well-being', 'guest-bloggers', 'ms-publications', 'videos', 'multiplesclerosis-net', 'sharkfest', 'surveys', 'swim-for-ms', 'the-motivator' ) ) ){
      $single_template = dirname( __FILE__ ) . '/CUSTOM-POST-BLOG-POST.php';

 }
 return $single_template;

}

add_filter( "single_template", "get_custom_cat_template" ) ;