Sending object to server, encoding problems (json,stringify,json_encode)?

Question

PHP

Good day friends, I send an object to the server:
javascript

var json = [{"key":"29","value":"Country"},{"key":"30","value":"4,3,5"}];

var xhr = new XMLHttpRequest();			
xhr.open('POST', 'read.php');
xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');			
xhr.send('json=' + JSON.stringify(json)); //[{"key":"29","value":"Country"},{"key":"30","value":"4,3,5"}];

PHP

<?php
if (!isset($_POST['json'])) exit;
$fn = 'myjson.json';
file_put_contents($fn, json_encode($_POST['json']));

the file is written with the following value:

"[{\"key\":\"29\",\"value\":\"Country\"},{\"key\":\"30\",\"value\":\"4,3,5\"}]"

Where and what needs to be changed, what would be like in the original, without slashes (\)
Thank you in advance!

Answer 1

json_encode remove
why do json_encode what is already json?