PHP variable declared global doesn't work within function - php

Question

I am using a HTTP query string to pass an id. The variable I assign it to works perfectly for the queries immediately following. However, it doesn't work within any of the functions which I define in the same file, although I declared the variable as global.
$circleID = $_GET['id'];
$circleID works well for this query:
// Retrieve circle data
$circleDataResult = mysqli_query($connection," SELECT name, description
FROM circle
WHERE circleID = '$circleID' ");
$circleData = mysqli_fetch_array($circleDataResult);
$circleName = $circleData['name'];
$circleDesc = $circleData['description'];
It doesn't work within the following function though. $circleID seems to be empty in this context:
if(isset($_POST['action']) && !empty($_POST['action'])) {
$action = $_POST['action'];
switch($action) {
case 'removeUser' : removeUser(); break;
case 'makeAdmin' : makeAdmin(); break;
case 'revokeAdmin' : revokeAdmin(); break;
case 'makeOwner' : makeOwner(); break;
}
}
function removeUser(){
global $connection;
global $circleID;
$thisUserID = $_POST['id'];
$removeUserFromCircle = " DELETE
FROM circle_participants
WHERE circleID = '$circleID' AND userID = '$thisUserID' ";
if (mysqli_query($connection, $removeUserFromCircle)) {
echo "You removed " . getName($thisUserID) . " from this circle";
} else {
echo "Error deleting record: " . mysqli_error($connection);
}
}
Apologies if this is a trivial question. I am new to php and spent a considerable amount of time trying to solve this, but I'm stuck.

Answer 1

In order to sum up the answer for anyone else encountering the problem:
It seems like the id value I wanted to retrieve via GET from the HTTP query string value was overwritten/set to null by the POST request, as user3411846 pointed out. Thus, when the code was executed via AJAX, circleID was set to null.
Using session variables in conjunction with if(isset){} solved the problem!
This is the bit of code I changed:
if(isset($_GET['id'])){
$_SESSION['circleid'] = $_GET['id'];
}
instead of:
$circleID = $_GET['id'];
And within the function:
function removeUser(){
...
$circleID = $_SESSION['circleid'];
...
}
instead of:
function removeUser(){
...
global $circleID;
...
}

Answer 2

See why this will not work
Suppose that you have sent an get request to a page
$a = $_GET['variable'];
echo $a ; // this will echo the variable
exit;
Now you are making once again the post request to this page
$a=$_POST['variable'];
echo $a; // will print data if there exist data in the given variable
exit;
Now since you want to access the previous get data in the post data you have to save the get data in the session since on the next request all the data from the previous request will be lost
so in starting of the page
session_start()
$circleid = $_GET['id'];
$_SESSION['cirlceid'] = $circleid;
and in the remove function
function removeUser(){
global $connection;
$circleID = $_SESSION['circleid']
$thisUserID = $_POST['id'];
$removeUserFromCircle = " DELETE
FROM circle_participants
WHERE circleID = '$circleID' AND userID = '$thisUserID' ";
if (mysqli_query($connection, $removeUserFromCircle)) {
echo "You removed " . getName($thisUserID) . " from this circle";
} else {
echo "Error deleting record: " . mysqli_error($connection);
}
}