How to implement firebase cloud messaging in server side?

I added notification object in my json. I found out that in my remoteMessage.getNotification().getBody() it returns null that's why it doesn't receive any notification send by my cron.

Edit

Here's my json object

$message = array(
            'registration_ids' => $registrationIDs,
            'notification' => array(
                                    "title" => $id, 
                                    "body" => $messageText,
                                    "icon" => "name_of_icon" ),
            'data' => array(
                    "message" => $messageText,
                    "id" => $id,
            ),
    );

I am not able to comment directly in the selected answer. Take in consideration that the json message changes according to what you want/need to receive in the recipient os (android/ios).

e.g for android to get the notifications when the app is in background you need to add the data json in your request.

For iOS the data key is not needed, but you must have the notification key along with isContentAvailble flag set to true and priority set to high.

In the selected answer as valid, you use registration_ids key should be only used when you are sending the notification to more than one device, maybe you should take a look to the topics notifications.

Apart from changing the url to following:

https://fcm.googleapis.com/fcm/send

You also have to change the way you send request data:

 Content-Type:application/json
 Authorization:key=AIzaSyZ-1u...0GBYzPu7Udno5aA

 {
   "to" : "bk3RNwTe3H0:CI2k_HHwgIpoDKCIZvvDMExUdFQ3P1...", // "to" replaces "registration_ids" of gcm in fcm
   "data" : {
   ...
  },
}

Check out this complete guide.