InvalidRegistration for FCM in php? - php

Question

I am receiving an [InvalidRegistration] error in send_fcm message
Following is the code that I have used:
<?php
define("GOOGLE_SERVER_KEY", "AAAAgTwiaRM:APA91bFyOiLtJKim7FIKpW...Mk6rR4xlTNDr1Ia0sJmerecuD6U1c6xsQh8PZdKz");
function send_fcm($message, $id) {
$url = 'https://fcm.googleapis.com/fcm/send';
$headers = array (
'Authorization: key='.GOOGLE_SERVER_KEY,
'Content-Type: application/json'
);
$fields = array (
'data' => array ("message" => $message),
'notification' => array ("body" => $message)
);
if(is_array($id)) {
$fields['registration_ids'] = $id;
} else {
$fields['to'] = $id;
}
$fields['priority'] = "high";
$fields = json_encode ($fields);
$ch = curl_init ();
curl_setopt ( $ch, CURLOPT_URL, $url );
curl_setopt ( $ch, CURLOPT_POST, true );
curl_setopt ( $ch, CURLOPT_HTTPHEADER, $headers );
curl_setopt ( $ch, CURLOPT_RETURNTRANSFER, true );
curl_setopt ( $ch, CURLOPT_POSTFIELDS, $fields );
$result = curl_exec ( $ch );
if ($result === FALSE) {
die('FCM Send Error: ' . curl_error($ch));
}
curl_close ( $ch );
return $result;
}
$message_status= send_fcm("test", "sketch_up");
echo $message_status;
?>
After I execute the above code, I find the error as mentioned below
{"multicast_id":7765951050747928269,"success":0,"failure":1,"canonical_ids":0,"results":[{"error":"InvalidRegistration"}]}
What could be the root cause - how may I fix?