Update variable value in function - php

Question

I am updating a private attribute of a class that has a singleton pattern implemented and I am changing its value in one function of a class but in other function its value is not changing. Here is my class:
class notifications
{
private static $contacts = array(
"[phone]" => array(
"status" => "pending"
),
"[phone]" => array(
"status" => "pending"
)
);
private $next = false;
private $log;
// Singleton instance
private static $instance;
private function __construct()
{
$this->log = fopen("log.txt", "a+") or die("Unable to open file!");
}
// getInstance method
public static function getInstance()
{
if(!self::$instance) {
self::$instance = new self();
}
return self::$instance;
}
// Send Notification via Call
public function notifyCall($message)
{
$client = new Services_Twilio($this->account_sid, $this->auth_token);
try {
$keys = array_keys(notifications::$contacts);
foreach ($keys as $key) {
fwrite($this->log, "sending notification to ".$key."\n");
// Sending call logic here
$this->next = false;
while(!$this->next) {
echo $this->next;
if($this->next === -1)
die;
}
}
}
catch(Exception $e) {
fwrite($this->log, $e->getMessage());
throw new Exception($e->getMessage());
}
fclose($this->log);
http_response_code(200);
}
// Set status for call
public function setStatus($recipient, $status)
{
$recipient = str_replace(" ", "+", $recipient);
$file = fopen("status.txt", "a+");
fwrite($file, $recipient." : ".$status."\n");
$keys = array_keys(notifications::$contacts);
notifications::$contacts[$recipient]["status"] = $status;
if(notifications::$contacts[$recipient]["status"] === "in-progress") {
fwrite($this->log, "notifying next contact");
$this->next = -1;
}
else if(notifications::$contacts[$recipient]["status"] === "busy" || notifications::$contacts[$recipient]["status"] === "no-answer") {
fwrite($this->log, "Next!");
$this->next = true;
}
foreach ($keys as $key) {
echo "$key: ".notifications::$contacts[$key]['status']."\n";
}
fclose($file);
}
}
Here is how i use this class
require_once("notifications.php");
$recipient = $_POST["To"];
$status = $_POST["CallStatus"];
$notification = notifications::getInstance();
$notification->setStatus($recipient, $status);
Please note that both classes and status update are on separate files and they have different URLs but the contacts and their statuses are shared among both files
What I get as output in my log file is that it started sending call to first contact and the status is received as well and recorded in setStatusfunction as well a next is printed in log file but on the sendNotificationCall function it does not actually end while loop nor it send notification to next contact.
Can anyone please guide me what I am dong wrong here? I also tried making $next as static like $contacts but it did not worked as well so I am confused about the mistake.