在PHP和DIV HTML之间传递变量

<form action="example.php" method="post">
  <input type="text" name="variable">
  <input type="submit" value="Submit">
</form>

<?php
  $variable = $_POST['variable'];
  echo $variable;
?>

<div id="result"></div>

<script>
  var variable = "Hello, World!";

  var xhr = new XMLHttpRequest();
  xhr.onreadystatechange = function() {
    if (xhr.readyState === 4 && xhr.status === 200) {
      document.getElementById("result").innerHTML = xhr.responseText;
    }
  };
  xhr.open("GET", "example.php?variable=" + variable, true);
  xhr.send();
</script>

<?php
  $variable = $_GET['variable'];
  echo $variable;
?>

<?php
  session_start();
  $_SESSION['variable'] = "Hello, World!";
?>

HTML文件：
<div>
  <?php
    session_start();
    echo $_SESSION['variable'];
  ?>
</div>