Push notification sent from Laravel is received by Android device, but isn't received by iOS device

Question

This is my code to send push notifications to mobile app. The code is in Laravel/PHP

$firebaseUrl            = FIREBASE_API_URL;
$firebaseApiKey         = FIREBASE_API_KEY;

$registrationIds        = $deviceIdArray;
$notification           = array(
                            'title'      => $notificationTitle, 
                            'message'    => $notificationMessage,
                            'icon'       => BASE_URL . 'assets/images/icon/' . $notificationCalamity . '.png',
                            'body'       => $notificationMessage
                            //"url"   =>$firebaseUrl
                        );
$data                   = array (
                            'message'           => $notificationMessage,
                            'additionalData'    => $notificationUrl,
                            'url'               => $notificationUrl,
                            'hazard'            => $notificationCalamity,
                            'quake_id'          => $notificationQuakeId
                        );

$fields = array (
    'registration_ids'  => $registrationIds,
    'notification'      => $notification,
    'data'              => $data 
);       

//header includes Content type and api key
$headers = array(
    'Content-Type:application/json',
    'Authorization:key=' . $firebaseApiKey
);

//print_r($headers);

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $firebaseUrl);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($fields));
$curlResult = curl_exec($ch);

if ($curlResult === FALSE) {
    // if curl response is false, it has failed;
    $notificationFcmMessage = 'FCM Send Error: ' . curl_error($ch);
} else {
    // if curl response is success, it has been passed 
    $notificationFcmMessage = $curlResult;

    $result = json_decode($curlResult, true);
    if(isset($result) && !empty($result) && $result !== null) {
        if(intval($result['success']) > 0) {
            $notificationSentStatus = SMALL_CHAR_YES;
        }
    }
}
curl_close($ch);

The code base is sending push notifications to android devices successfully. However, iOS devices are not receiving any notification. I think the issue is with the notification object that I am building. Maybe, something to do with the keys.

How can I fix this?