Sending json from forms

mail.php

<?php
  $json = file_get_contents('php://input');
  $data = json_decode($json, true);
  $errors = array();

  // Form params variables

  $param1 = '';
  $param2 = '';

  if (isset($data['param1'])) {
    $param1 = $data['param1'];
  } else {
    array_push($errors, 'Need param1.');
  }
  
  if (isset($data['param2'])) {
    $param2 = $data['param2'];
  } else {
    array_push($errors, 'Need param2.');
  }

  if (count($errors) != 0) {
    $errors_messages = '';
    foreach($errors as $error) {
      $errors_messages = $errors_messages.$error.' ';
    }
    echo $errors_messages;
  } else {
    $html = '
      <h3>Message header.</h3>
      <p>Param1: '.$param1.'</p>
      <p>Param2: '.$param2.'</p>';
    $to  = "<mail@gmail.com>";
    $subject = "Message subject.";
    $message = $html;
    $headers  = "Content-type: text/html; charset=utf-8 \r\n";
    $headers .= "From: Mail from <noreply@example.com>\r\n";
    $headers .= "Reply-To: reply-to@example.com\r\n";
    mail($to, $subject, $message, $headers);
    echo 'Message send!';
  }
 ?>

req.js

var data = {
  param1: 1,
  param2: 2
}
var xhr = new XMLHttpRequest()
xhr.open('POST', 'mail.php', true)
xhr.setRequestHeader('Content-Type', 'application/json; charset=utf-8')
xhr.send(JSON.stringify(data))
xhr.onreadystatechange = function () {
  if (xhr.readyState != 4) return
  if (xhr.status === 200) {
    console.log(xhr.responseText )
  } else {
    console.log(xhr.status + ': ' + xhr.statusText )
  }
}