Почему PHP не принимает JSON?

function sendInfo(json) {
                var someObj = JSON.stringify(json);
              // console.log(someObj);
                var xhr = new XMLHttpRequest();               
                xhr.open('POST', 'foo.php',true);
                 xhr.setRequestHeader('Content-Type','application/json; charset=utf-8');                
                xhr.send(someObj);               
                xhr.onreadystatechange = function () {
                    if (this.readyState === 4) {
                        if (this.status === 200)
                             console.log('ok');
                        else
                            console.log('ajax error');
                    }
                };
            }

Request URL: http://site/foo.php
Request Method: POST
Status Code: 200 OK

<?php
error_reporting(~0);
ini_set('display_errors', 1);
var_dump($_POST);
$request_body = file_get_contents('php://input');
echo $request_body;
$post = json_decode($request_body);
print_r($post);
print_r(json_decode(trim(file_get_contents('php://input')),1));