How to redirect a person with a get request?

Question

PHP

A person entering the cart goes to page ../page_order.php?action=onclick
If he goes to page ../page_order.php, the page will be displayed clumsily.
How to make it so that when going to ../page_order.php it would be transferred to ../page_order.php?action=onclick via a GET request, or maybe somehow differently?

Answer 1

if I don't have a variable, then it takes the default value.

Answer 2

At the beginning of the script we write:

if ( ! isset($_GET['action']) || $_GET['action'] !== 'onclick') {
    header('Location: ' . PHP_SELF . '?action=onclick'); exit(0);
}