Channelid add notification php

Question

I am sending a notification with the android php file, right now it is empty because I need to add channelID. How do I define Array to channelID? Empty notification going because of no channelID

<?php 
sendNotification($title,$message,"android");

function sendNotification($title="",$message="",$type="android") {
    $path_to_fcm = 'https://fcm.googleapis.com/fcm/send';
    $server_key = "..";

    if($type=="ios"){
        // IOS KEY
        $finalKey = array('..');
    }else{
        // ANROID KEY
        $finalKey = array('..');
    }

    $headers = array(
        'Authorization:key=' .$server_key, 
                'Content-Type : application/json');

    $fields = array('to'  => '/topics/...', 'notification'=>array('title'=>$title, 'message'=>$message));


    $payload = json_encode($fields);

    $curl_session = curl_init();
    curl_setopt($curl_session, CURLOPT_URL, $path_to_fcm);
    curl_setopt($curl_session, CURLOPT_POST, true);
    curl_setopt($curl_session, CURLOPT_HTTPHEADER, $headers);
    curl_setopt($curl_session, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($curl_session, CURLOPT_SSL_VERIFYPEER, false);
    curl_setopt($curl_session, CURLOPT_IPRESOLVE, CURL_IPRESOLVE_V4);
    curl_setopt($curl_session, CURLOPT_POSTFIELDS, $payload);
    curl_setopt($curl_session, CURLOPT_ENCODING, "");
    curl_setopt($curl_session, CURLOPT_MAXREDIRS, 10);
    curl_setopt($curl_session, CURLOPT_TIMEOUT, 30);
    curl_setopt($curl_session, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_1);
    curl_setopt($curl_session, CURLOPT_CUSTOMREQUEST, "POST");
    $result = curl_exec($curl_session);
    if ($result === FALSE) {
        die('Oops! FCM Send Error: ' . curl_error($curl_session));
    }
    curl_close($curl_session);
    return json_decode($result,TRUE);

}