\Dom\DocumentFragment::getElementById() is not supported

Description

The following code:

<?php
$d = \Dom\HTMLDocument::createFromString('<p id="a">b</p><p id="a">c</p>');
$df = $d->createDocumentFragment();
$df->getElementById('a');

Resulted in this output:

  Error  Call to undefined method Dom\DocumentFragment::getElementById().

But I expected this output instead:

(no output)

This method is present in the DOM spec: https://developer.mozilla.org/en-US/docs/Web/API/DocumentFragment/getElementById and the similar method ::querySelector is implemented; I can't tell if querySelector actually uses the ID index though or does a linear scan through the fragment. (Aka, this method should not just be implemented, it is expected to have O(1) performance.)

PHP Version

PHP 8.4.13 (cli) (built: Oct  1 2025 20:34:15) (NTS)
Copyright (c) The PHP Group
Built by Debian
Zend Engine v4.4.13, Copyright (c) Zend Technologies
    with Zend OPcache v8.4.13, Copyright (c), by Zend Technologies

Operating System

Ubuntu 24.04.1

首先查看链接的 pull request，以及 issue 中提到的现有 Dom\DocumentFragment::querySelector 实现。参考 DOM 规范中关于 getElementById() 语义的说明，包括重复 ID 和预期的查找行为；完成的标准是该方法可用，并按规范返回匹配的 fragment 元素。