I am trying to pass a parameter to a WordPress site using a URL - for instance:

`www.fioriapts.com/?ppc=1` will be the URL. 

I am intending to write a function in the functions.php file but the mechanics of how to extract a parameter in WordPress is beyond me. I am finding a lot of examples on how to add a parameter to a URL using the function `add_query_arg()` but have found nothing on how to extract a parameter. Thanks in advance for any help.