set my own variables

Why not use something like:

$car_id = get_cat_ID('car');

Okay, so if I set this code on functions.php, could I later call it on the other theme files? for example when I use:

 if ( post_is_in_descendant_category( $car_id ) ) {} 

Or I have to declare it in every theme file?

Thanks for the quickly reply.

You should be able to put a function in your theme’s functions.php and ‘call’ that function in your other theme template files.

Michael I really appreciate your attention, but I have to ask you to be a little more specific. I have tried to find material on the net on how to set variables on functions, but I could not find anything. I did some try and error as well, again no luck, that is why I came to the forum.

Will that work?

function cats_ids(){
$car_id = get_cat_ID('car');
$airplaine_id = get_cat_ID('airplane');
}

and get the variable using that on the other's theme files:
cats_ids();
if ( post_is_in_descendant_category( $car_id ) ) {}

Thank you

https://www.google.com/search?q=php+pass+values+to+functions

Also can look at WordPress builtin functions (template tags) and see how that is accomplished. Further reading on that:
https://codex.www.ads-software.com/How_to_Pass_Tag_Parameters

Hi Michael. Just saw your message. Will now read the links that you sent.

Again thank you for your fast replies.

For the record. If you have a variable that you want to use in the other file themes you can put something like that in your functions.php:

function car_id( $car = '2320' ) { return $car; }
function airplane_id( air$plaine = '2321' ) { return $airplane; }

To call the variable in some other theme file just use:

car_id()
airplane_id()

to accesses it.

to display use:

echo car_id()
echo airplane_id()