@ed
There’s a possible example near the bottom of https://developer.wordpress.org/reference/functions/is_user_logged_in/
I tried pasting it here, but it looked corrupted, so best to look at the example (kakashi-hatake) in the above page.
I don’t normally use either wordpress or php so I can’t vouch for it, or know whether it will work with the rest of the site, though the example looks fairly logical.