Field "to" must be a JSON string [Firebase] - php

Question

I'm trying to send notification using a cron job. I migrated from GCM to FCM. In my server side, I changed https://android.googleapis.com/gcm/send to https://fcm.googleapis.com/fcm/send and also updated on how to request the data changing registration_ids to to. Checking the json passed it is a valid json but I'm having an error Field "to" must be a JSON string. Is there anyway to solve this?
Here is my code
function sendNotificationFCM($apiKey, $registrationIDs, $messageText,$id) {
$headers = array(
'Content-Type:application/json',
'Authorization:key=' . $apiKey
);
$message = array(
'to' => $registrationIDs,
'data' => array(
"message" => $messageText,
"id" => $id,
),
);
$ch = curl_init();
curl_setopt_array($ch, array(
CURLOPT_URL => 'https://fcm.googleapis.com/fcm/send',
CURLOPT_HTTPHEADER => $headers,
CURLOPT_POST => true,
CURLOPT_RETURNTRANSFER => true,
CURLOPT_POSTFIELDS => json_encode($message)
));
$response = curl_exec($ch);
curl_close($ch);
return $response;
}

Answer 1

Try explicitly converting $registrationIDs to string.
$message = array(
'to' => (string)$registrationIDs,
'data' => array(
"message" => $messageText,
"id" => $id,
),
);
Edited Answer
The 'to' parameter requires a string - which is the recipient of the message.
The $registrationIDs could be passed a separate parameter (as string array) to 'registration_ids'
Edit your code to something like this:
$recipient = "YOUR_MESSAGE_RECIPIENT";
$message = array(
'to' => $recipient,
'registration_ids' => $registrationIDs,
'data' => array(
"message" => $messageText,
"id" => $id,
),
);
Where $recipient is
a registration token, notification key, or topic.
Refer this:
Firebase Cloud Messaging HTTP Protocol

Answer 2

Try making to as a string:
Content-Type:application/json
Authorization:key=AIzaSyZ-1u...0GBYzPu7Udno5aA
{
"to" : "bk3RNwTe3H0:CI2k_HHwgIpoDKCIZvvDMExUdFQ3P1...",
"data" : {
...
},
}

Answer 3

// if you are using an array like
$fcm_ids = array();
$fcm_ids[] = "id_1";
$fcm_ids[] = "id_2";
$fcm_ids[] = "id_3";
//.
//.
//.
$fcm_ids[] = "id_n";
// note: limit is 1000 to send notifications to multiple ids at once.
// in your messsage send notification function use ids like this
$json_data = [
"to" => implode($fcm_ids),
"notification" => [
"title" => $title,
"body" => $body,
],
"data" => [
"str_1" => "something",
"str_2" => "something",
.
.
.
"str_n" => "something"
]
];

Answer 4

I had a similar problem. It turns out when I retrieved the Registration Token from my Firebase database (using this Firebase PHP Client), it was returned with double quotes at the beginning and at the end of the token. So I had to remove the first and last characters from the token before I could use it. The code below solved my problem
substr($registrationToken, 1, -1)