Send FCM notification silently in to app with PHP

function send_notification($device_id,$content,$title){

 $url = 'https://fcm.googleapis.com/fcm/send';
 $fields = array (
   'data' => array (
     "title" => $title,
     "detail" => $content
   ),
   'to' => $device_id
 );
 
 $fields = json_encode ( $fields );
 $headers = array (
   'Authorization: key='."xxxxxx-xxxxxxxxxxxxxxx",
   'Content-Type: application/json'
 );
 
 $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_POSTFIELDS, $fields );
 
 $result = curl_exec ( $ch );
 curl_close ( $ch );
}