Get url variable from AJAX in PHP - php

Question

I've looked at a couple of related topics here on Stackoverflow such as this answer, but I can't seem to get this one correct – it could be something particular in my code, but I don't think that should be the case, thus I'm seeking your advice.
My goal is to pass index.php?key=123testID through the client.js script below to my update-check.php in order to long-poll the txt file with the corresponding name (in this example 123testID.txt). If there's an other way for update-checker to grab the variable, that is also acceptable.
I currently have working functionality for long-polling the .txt data – I simply want to add the functionality to dynamically load the txt file with the same name as the URL variable.
At the moment my code looks like this:
client.js
function getContent(timestamp)
{
var queryString = {'timestamp' : timestamp};
$.ajax(
{
type: 'GET',
url: 'update-check.php',
data: queryString,
success: function(data){
// put result data into "obj"
var obj = jQuery.parseJSON(data);
// put the data_from_file into #response
$('#response').html(obj.data_from_file);
// call the function again, this time with the timestamp we just got from server.php
getContent(obj.timestamp);
}
}
);
}
// initialize jQuery
$(function() {
getContent();
});
update-check.php
<?php
// set php runtime to unlimited
set_time_limit(0);
// how can I load a variable insead of a static txt file?
$data_source_file = "stories/123testID.txt";
// main loop
while (true) {
// if ajax request has send a timestamp, then $last_ajax_call = timestamp, else $last_ajax_call = null
$last_ajax_call = isset($_GET['timestamp']) ? (int)$_GET['timestamp'] : null;
clearstatcache();
// get timestamp of when file has been changed the last time
$last_change_in_data_file = filemtime($data_source_file);
if ($last_ajax_call == null || $last_change_in_data_file > $last_ajax_call) {
$data = file_get_contents($data_source_file);
$result = array(
'data_from_file' => $data,
'timestamp' => $last_change_in_data_file
);
// encode to JSON, render the result (for AJAX)
$json = json_encode($result);
echo $json;
// leave this loop step
break;
} else {
sleep( 1 );
continue;
}}?>