How to substitute parameters from UTM into links on the site?

Question

URL Handling

Tell me how to substitute parameters from UTM into links on the site?
That is, the task is as follows:
My client visits the site example.com/?utm_source=utm and the site has a "Buy" button that leads to the page 2example.com/order.
How can I make it so that if the client clicks on the link example.com/?utm_source=utm , then his button shows: 2example.com/order?utm_source=utm ?

Answer 1

if (isset($_GET['utm_source'])){
   $url_add = '?utm_source='.$_GET['utm_source'];
}

else {
  $url_add = false;
}

<a href="http://2example.com/order<?= $url_add?>">Купить</a>