添加LIBXML_HTML_NOIMPLIED后DOMDocument修复HTML出现h3标签异常

拆分HTML文本时可能出现标签破损问题，原本使用以下DOMDocument修复代码：

$html_intro = '<h3>Title</h3><p>Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.</p>';
$doc = new DOMDocument();
$doc->substituteEntities = false;
$content = mb_convert_encoding($html_intro, 'html-entities', 'utf-8');
@$doc->loadHTML($content, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);
$html_intro = $doc->saveHTML();

为避免自动添加<body>元素，添加了LIBXML_HTML_NOIMPLIED参数后，修复功能失效。执行var_dump($html_intro)得到以下结果：

string(613) "<h3>Title<p>Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.</p></h3>"

问题原因 #

HTML规范中，<p>这类块级元素不能嵌套在<h3>等标题标签内部。正常情况下，libxml加载HTML时会自动添加隐含的<html>和<body>元素，解析器会将<h3>闭合后，把<p>作为<body>的子元素，保证结构合规。

但添加LIBXML_HTML_NOIMPLIED参数后，libxml不再自动生成顶层的<body>容器，解析器失去了同级元素的承载容器，只能将后续的<p>元素强行嵌套进前一个<h3>标签内，导致出现不符合规范的嵌套结构。

合理解决方案 #

不需要通过LIBXML_HTML_NOIMPLIED禁用隐含元素，而是加载HTML后提取<body>内部的内容，这样既能避免输出<html>/<body>标签，又能让解析器正常校正结构：

$html_intro = '<h3>Title</h3><p>Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.</p>';
$doc = new DOMDocument();
$doc->substituteEntities = false;
$content = mb_convert_encoding($html_intro, 'html-entities', 'utf-8');
@$doc->loadHTML($content, LIBXML_HTML_NODEFDTD);

// 提取body节点下所有子元素的HTML内容
$html_intro = '';
$bodyNode = $doc->getElementsByTagName('body')->item(0);
foreach ($bodyNode->childNodes as $node) {
    $html_intro .= $doc->saveHTML($node);
}

var_dump($html_intro);

执行后会输出正确的结构，且不带多余的顶层标签。

内容的提问来源于stack exchange，提问作者maidan