在php中获取<select>标签的选定索引值

<form method="POST">
 <select name="select_index">
   <option value="0">选项1</option>
   <option value="1">选项2</option>
   <option value="2">选项3</option>
  </select>
 <input type="submit" value="提交">
</form>

<?php
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
  $selected_index = $_POST['select_index'];
  echo "选定的索引值为：" . $selected_index;
}
?>

 <option value="0">选项1</option>
 <option value="1">选项2</option>
 <option value="2">选项3</option>
</select><script>
  document.getElementById('select_index').addEventListener('change', function() {
    var selected_index = this.value;
    console.log("选定的索引值为：" + selected_index);
  });
</script>