php - Calling Firebase Cloud Messaging from server does not work -

i developed firebase web app server. sending notifications , seems okay in android app no notifications received. if send notification firebase console comes android app. want send web app.

here php code web app:

public function sendmessage($data,$target=null){         //fcm api url         $url = 'https://fcm.googleapis.com/fcm/send';         //api_key available in firebase console -> project settings -> cloud messaging -> server key         $server_key = 's6gzacuhibs6gzacaizasyjtkqlzl3fcuhibuhibs6gza8a';          //$fields = array();         //$fields['data'] = $data;         if(is_array($target)){             $fields['registration_ids'] = $target;         }              $fields = array(         'notification' => array('title' => 'working good', 'body' => 'that want'),         'data' => array('message' => "melaapa")     );            //header content_type api key         $headers = array(             'content-type:application/json',           'authorization:key='.$server_key         );          $ch = curl_init();         curl_setopt($ch, curlopt_url, $url);         curl_setopt($ch, curlopt_post, true);         curl_setopt($ch, curlopt_httpheader, $headers);         curl_setopt($ch, curlopt_returntransfer, true);         curl_setopt($ch, curlopt_ssl_verifyhost, 0);         curl_setopt($ch, curlopt_ssl_verifypeer, false);         curl_setopt($ch, curlopt_postfields, json_encode($fields));         $result = curl_exec($ch);         if ($result === false) {             die('fcm send error: ' . curl_error($ch));         }         curl_close($ch);         var_dump($result);         return $result;         } 

the output of

var_dump($result)

is

string(3) "to "

what should reponse working firebase server or how can understand firebase server working fine?