Find <p> that contains image(s) and change style of that <p> - php

Question

I'm parsing Wordpress post HTML through PHP. I want all images to be centered. This alone is easy enough, however, I also want images on the same line to be centered together. In order to do this I need to apply the attribute class="image-content" to the <p> block.
How do I do this with PHP?
This is what the post would look like in the editor:
And this is the HTML that Wordpress provides for this post:
<p>Single line paragraph.</p>
<p>
<a href="image.png">
<img class="alignnone wp-image-39 size-thumbnail" src="image.png" width="150" height="150" />
</a>
</p>
<p>
Multi line paragraph which is a multi line paragraph
which is a multi line paragraph which is a multi line
paragraph which is a multi line paragraph which is a
multi line paragraph which is a multi line paragraph
which is a multi line paragraph which is a multi line
paragraph which is a multi line paragraph which is a
multi line paragraph which is a multi line paragraph
which is a multi line paragraph which is a multi line
paragraph which is a multi line paragraph which is a
multi line paragraph which is a multi line paragraph.
</p>
<p>
<a href="image.png">
<img class="alignnone wp-image-39 size-thumbnail" src="image.png" width="150" height="150" />
</a>
<a href="image.png">
<img class="alignnone wp-image-39 size-thumbnail" src="image.png" width="300" height="300" />
</a>
</p>
<p>End of post.</p>

Answer 1

You can do this with DOMDocument, xpath and a simple replacement.
$parse = new \DOMDocument();
$parse->loadHTML($html);
$xpath = new \DOMXpath($parse);
$images = $xpath->query('//p//img');
$re = "/(.*)/";
$subst = "$1 image-content";
foreach ($images as $image) {
$class = preg_replace($re, $subst, $image->getAttribute('class'), 1);
$image->setAttribute('class',$class);
}
$htmlFinal = $parse->saveHTML();
EDIT
If you want to attach the class to the containing p Element, you can use it like this:
$parse = new \DOMDocument();
$parse->loadHTML($html);
$xpath = new \DOMXpath($parse);
$ps = $xpath->query('//p');
foreach ($ps as $p) {
if ($p->getElementsByTagName('img')->length > 0) $p->setAttribute('class', 'image-content');
}
$htmlFinal = $parse->saveHTML();
If the p tags may have a class set before parsing the Dom, you should combine those two examples to add the new class instead of only setting it.