Q:

How do I update a value of a variable in PHP?

Context:
I am working in MemberMouse (a WordPress Membership Plugin), and I am trying to write a script that updates a member's status to "Cancelled" after a Refund has been issued on their account.

I've been provided with a sample script that simply pulls/provides the users information: https://dl.dropboxusercontent.com/u/265387542/files/payment_notification_script.php 

Problem: 
I want to take $status = $_GET["status"]; and update it to "Cancelled". How can I achieve this? Is there a way to $_POST a status value back to MemberMouse, or to set $status equal to something else? 
(I am a novice with PHP). Here's my attempt below:
...
// ---- PERFORM ACTION BASED ON EVENT TYPE ----
switch($eventType)
{       
    case $REFUND_ISSUED:
        // set status to cancel
        mm_member_status_change("Cancelled");

        break;
}

A:

Since you say you are new to PHP, here are some points you may not know.
1.  $status = $_GET["status"]; may not be doing what you think it is doing.
this line is saying two things.

it is declaring a variable $status 
it is setting that
    variable to the URL variable $_GET["status"];

What is a URL variable?
in this case, it would look something like this in the URL.

www.mysitename.com/index.php?status=canceled

so, if you want to change the "status", you simply redefine the variable like so.
$status="canceled";

