I don't understand why you want to make your code somewhat open to issues in the future.
Is it so much to use:
$firstname = $_GET['firstname'];
this can be made to work very nicely into your code.
You can insert it right into your HTML:
<HTML>
Hi <?= $_GET['firstname'] ?>,
Welcome...
</HTML>
or declare them on the top of your code.
Most good Webhost have ugraded there PHP versions. If you make your codes without this feature you could find yourself in a bit of trouble if they dont allow you to edit you PHP.INI file.
To each is own. Do what you must... :-)
