How to get URL from another page?

Question

PHP

Good afternoon, I am writing a testing system, there are tests on the main page, after the user clicks on this test, the value test=1 (test id) appears in the url
I want to send the test id to the database, I have the function for sending the result in functions.php
did $_GET['test'] but it can't find such a value.
I did the same in the main page, everything is displayed, how to transfer the URL to another file?