You can use the isset()
function - this is a very common thing to use if you need to check whether input values have been provided or not.
In the simplest case, just exit the script if the value isn't set:
if (!isset($_GET["abc"])) exit();
Documentation: https://www.php.net/manual/en/function.isset.php
P.S. $_SERVER['REQUEST_URI']
provides the whole URI following the domain, including the path and the whole querystring. So you could pick that apart to get the value you want, but it's pointless because $_GET["abc"]
will take you straight to it. See this answer for an example of the difference.