PHP DOMDocument extract elements and create new document

HTML:

<head>
    <link rel="preload" href="/_next/list.js" as="script">
    <!-- ... other link elemens -->
    <style data-styled="" data-styled-version="4.2.0"></style>
</head>

Task:

extract all link and style tags from head and update href attributes with host path.
create new html string element (piece of html) that we can output in our main html template.

PHP:

$dom = new DOMDocument();
$dom->loadHTML($stringBody);
$xpath = new DOMXPath($dom);
$headItems = $xpath->query("//head/link[@rel='preload' or @rel='stylesheet'] | //head/style");

// now I want to create html string with updated attributes, but I'm lost here..
$head = new DOMDocument();
foreach ($headItems as $headNode) {
    $headNode->setAttribute('href', $host . $headNode->getAttribute('href'));
}
$links = $head->saveHTML($headNode);
echo $links; // echo html link tags

Solution

Try this:

<?php

$host = 'example.com';

$stringBody = '<head>
    <link rel="preload" href="/_next/list.js" as="script">
    <!-- ... other link elemens -->
    <style data-styled="" data-styled-version="4.2.0"></style>
</head>';

$dom = new DOMDocument();
$dom->loadHTML($stringBody);
$xpath = new DOMXPath($dom);
$headItems = $xpath->query("//head/link[@rel='preload' or @rel='stylesheet'] | //head/style");

$links = [];

foreach ($headItems as $headNode) {
    if ($headNode->hasAttribute('href')) {
        $headNode->setAttribute('href', $host . $headNode->getAttribute('href'));
    }
    $links[] = $headNode->ownerDocument->saveHTML($headNode);
}

print_r($links);

Output

Array
(
    [0] => <link rel="preload" href="example.com/_next/list.js" as="script">
    [1] => <style data-styled="" data-styled-version="4.2.0"></style>
)