解析POST发送的JSON数据时，PHP中的Strange Object输出

I am a newbie with php and I aim to send JSON data from Particle Cloud (IoT cloud) to my Ubuntu 18.04 server with Apache2 and PHP7. I am writing php script that would parse JSON data but I have issues with accessing values of POSTed data

I have tried to follow different examples explaining how to convert $_POST array using json_decode() but I still have not figured out why it does not work

My file data.json is with JSON data

{"key1":"value1", "key2":"value2"}

my parser.php script is

<?php
$incoming = $_POST;
var_dump($incoming);

$data = json_decode(json_encode($var1), true);
var_dump($data);
?>

In Ubuntu 18.04 terminal I test dumped variables with curl

curl -v -X POST 'Content-Type: application/json;charset=UTF-8' -d data.json http://localhost/parser.php

My results

for $incoming
    array(1){
    ["{"key1":"value1","key2":"value2"}"]=>
    string(0) ""
    }

for $data
    object(stdClass)#1 (1) {
    ["{"key1":"value1","key2":"value2"}"]=>
    string(0) ""
    }

But I expect something like {"key1":"value1","key2":"value2"}, not array or Object in []

I can't access values by keys. For example,

var_dump($data['key1']);

brings NULL