Pull username in url to insert in link

Hello, I used to have a plugin that allowed me to create a snippet that would pull an affiliate tag from the url but the plugin stopped working. I was hoping that there would be an option to do the same thing that the old plugin did, but with this plugin?

Here is what I am trying to accomplish:

yourdomainname.com/?affid=username

I want to pull “username” from the url and insert that username into a link within the post content by using a shortcode from the plugin such as:

yourdomainname.com/[php-snippet]

When I try using the old code that I used with the old plugin, I am getting syntax errors. Here is the code:

?user=<?
if (empty($_GET[‘affid’]))
{
echo ‘1’;
}
else
{
echo $_GET[‘affid’];
}
?>

Would anyone have any suggestions as to how to make this work with this plugin?

Thanks so much for any help you can provide ??