Find that contains image(s) and change style of that

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.