How to get JSON from PHP?

Question

PHP

Hello. I've been struggling with the problem for about 2 hours, because I don't know PHP, but I need to do the task.
I have a button, when clicked, we get a new user using ajax

const getNewUser = () => {
  const xhr = new XMLHttpRequest();
  xhr.open('GET', 'https://randomuser.me/api/', true);
  xhr.onload = () => {
    if (xhr.status === 200) {
      const userData = JSON.parse(xhr.responseText);
      usersInformation(userData);
    } else {
      throw new Error(`Error: ${xhr.status} ${xhr.statusText}`);
    }
  };
  xhr.send();
};

I also have a function that forms an object with data about the user and calls a function to send this data to users.php, in order to then insert the data into an SQL table.

const usersInformation = (user) => {
  const userData = user.results[0],
    obj = {
      name: capitalizeFirstLetter(userData.name.first),
      lastname: capitalizeFirstLetter(userData.name.last),
      city: capitalizeFirstLetter(userData.location.city),
      phone: userData.phone,
      logoUrl: userData.picture.medium,
    };
  postData(obj);
};

const postData = (data) => {
  const stringifyData = JSON.stringify(data);
  const xhr = new XMLHttpRequest();
  xhr.open('POST', '../users.php', true);
  xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
  xhr.send(stringifyData);
}

Data is sent

XHR finished loading: POST "http://localhost:8080/users.php"

But I can't process them no matter how hard I tried.

<?php
$obj = file_get_contents('php://input');
echo($obj->name);
?>

And with the help , I $_POSTalso tried it in no way ((
Tell me how to do it, maybe I'm stupid somewhere?
Thanks in advance!