Can I print data sent by AJAX via a post request directly in a php page

I wanted to send a post request to a php page via ajax and then print it in the php page with $_POST, but I found that I couldn't jump to the page and then print out the data like I could with a form

<script>
    var xhr = new XMLHttpRequest();
    xhr.open("POST", "2.php", true);
    xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    xhr.onreadystatechange = function() {
        if (xhr.readyState === 4 && xhr.status === 200) {
            window.location.href = "2.php"
        }
    };
    xhr.send("username=John&email=john@example.com");
</script>

<?php
$username = $_POST['username'];
$email = $_POST['email'];

echo "Received username: " . $username . "<br>";
echo "Received email: " . $email;

In fact, I have done everything I can think of, but still can not achieve direct print data in php, should be my foundation is too poor, I hope that some master can teach me, write write

If you simply want the user to be redirected to the submission PHP, the simplest way is to simply submit a POST form:

<form id="my-account-form" method="POST" action="2.php">
  <input type="hidden" name="username" value="">
  <input type="hidden" name="email" value="">
</form>

...
...
...

<script>
// Use this function to fill in username + password and submit.
function submitAccount(username, password) {
  const form = document.getElementById('my-account-form');
  form.querySelector('input[name=username]').value = username;
  form.querySelector('input[name=password]').value = password;
  form.submit();
}
submitAccount('John', 'john@example.com');
</script>