在PHP中将嵌套数组转换为json，无需额外编码

I want to convert my array to a Json String and use it for send message using Firebase Cloud Messaging Server.
My array is :

$data = array(
                "to" => "cMbucIFcJMA:APA91bH_Wq0sPrMhTmhJ9gPJX7GUsRo...",
                "notification" => 
                    array(
                        "body" => "sadeq",
                        "title" => "test notification"
                    )
                );

and when I use json_encode($data) my back String is:
{"to":"cMbucIFcJMA:APA91bH_Wq0sPr.... ","notification":"Array"}
how can I convert this array to valid Json without have Array in my notification String???

My completely function is:

public function SendFromPost(){


                $data = array(
                    "to" => "cMbucIFcJMA:APA91bH_..... ",
                     "notification" => 
                    array(
                        "body" => "sadeq",
                        "title" => "test notification"
                    )
                );






                  $ch = curl_init();
                  curl_setopt($ch , CURLOPT_URL , "https://fcm.googleapis.com/fcm/send");
                  curl_setopt($ch , CURLOPT_POST , 1);
                  curl_setopt($ch , CURLOPT_POSTFIELDS , $data );
                  curl_setopt($ch , CURLOPT_RETURNTRANSFER , true);

                      $headers = [
                        'Content-Type: application/json 
',
                        'Authorization: key=AIzaS.......'
                      ];

                  curl_setopt($ch , CURLOPT_HTTPHEADER , $headers);

                  $server_output = curl_exec($ch);
                  curl_close($ch);


                 return var_dump(json_encode($data));

            }

My code is correct. but I don't know why my php do like this.

Your code works. You've got an extra ; that isn't valid after your nested array.

<?php
$data = array(
                    "to" => "cMbucIFcJMA:APA91bH_Wq0sPrMhTmhJ9gPJX7GUsRo...",
                    "notification" => 
                        array(
                            "body" => "sadeq",
                            "title" => "test notification"
                        )
                    );

var_dump(json_encode($data));

Yields

string(115) "{"to":"cMbucIFcJMA:APA91bH_Wq0sPrMhTmhJ9gPJX7GUsRo...","notification":{"body":"sadeq","title":"test notification"}}"