Access global variable in function


I have a first php file with a few variables in it that are defined outside of a function. I am trying to find a way to use these variables in a second php file in a function. Since I read that it is not recommended to use the global keyword I've tried to pass one of the variables as a parameter as a test based on one of the answers here but without success. Here's a simplified version of my code at the moment:

Vars.php with:

$test = 'test';  

Functions.php with:

include 'Vars.php';
function add_content($test){   echo 'This is a '. $test;  }  add_action( 'woocommerce_single_product_summary' 'add_content' 15 );  

I am not sure if this matters or not but as you can see this is for a wordpress website. If anyone knows how to do this