I'm sorry but I have no clue, this code works on dozens of websites without any issue, something is irregular on your website. Try to put it in the parent theme functions.php file, also try to remove the php end code ?>
I've done that. Have you had anyone using the code on WordPress 4.9?
I also installed a third party plugin for PHP code and it said the code causes a fatal error.
Please provide the fatal error message. This is not 4.9 compatibility issue. Something is irregular with your PHP, some extensions are not enabled or something else...
Also make sure you don't have function with identical name in other files. Function names should be unique.
Here's the error message provided to me by the plugin (it's the PHP inserter plugin):
"Sorry, but your code causes a "Fatal error", so it is not applied!
Please, check the code and try again."
I'm not sure what you mean by making sure I don't have a function with an identical name?
