lol.
all the javascript posted for this problem is unnecessary. upon the DOM being loaded/rendered, it's getting data that's known at the time of the request for the map2.php page (the value being sent to getBookedSeats.php is coming from a js variable that's being set to a php value echoed on the page.) this is just a roundabout wall of code and data churn.
here's a list of why this is not working -
1. you are making a POST request to getBookedSeats.php. the value won't be in any $_GET variable and adding an isset() won't make it work. all that did is hide the problem and caused the php code to be skipped over.
2. you are sending JSON encoded data to getBookedSeats.php. you would need to use the following to read and decode the data -
$json = file_get_contents('php://input');
$data = json_decode($json,true);
3. the value will then be in $data['screeningId'], because that's the name of the javascript variable holding the value that you are sending in the ajax fetch request, which is a value that is coming from php in the first place.