[PHP] Manipulate the DOM Using PHP

00-use-dom-document.php

<?php

use DOMDocument;

01-the-content-hook.php

<?php

add_filter('the_content', __NAMESPACE__ . '\\updateParagraphElements');

02-update-the-content.php

<?php

function updateParagraphElements($content) 
{
	/* If we're not on a single page or it's not the main query, 
	 * we won't do anything.
	 */
	if (!is_single() || !is_main_query()) {
		return $content;
	}

	// Make sure there is content to parse (and properly encode the HTML entities).
	$domDocument = new DOMDocument();
	$domContent = $domDocument->loadHTML(mb_convert_encoding($content, 'HTML-ENTITIES'));
	if (false === $domContent) {
		return $content;
	}
  
	$paragraphs = $domDocument->getElementsByTagName('p');
	if (0 === count($paragraphs)) {
		return $content;
	}

	// More to come...
}

04-full-code.php

<?php

namespace Acme;

use DOMDocument;

add_filter('the_content', __NAMESPACE__ . '\\updateParagraphElements');
/**
 * Updates the content by locating all of the `p` elements in the content,
 * then adds a class attribute of the post ID to them.
 *
 * The gist of the work is done in this function.
 *
 * @param string $content the post content
 *
 * @return string the updated post content with the aforemented markup.
 */
function updateParagraphElements($content) 
{
	/* If we're not on a single page or it's not the main query, 
	 * we won't do anything.
	 */
	if (!is_single() || !is_main_query()) {
		return $content;
	}

	// Make sure there is content to parse (and properly encode the HTML entities).
	$domDocument = new DOMDocument();
	$domContent = $domDocument->loadHTML(mb_convert_encoding($content, 'HTML-ENTITIES'));
	if (false === $domContent) {
		return $content;
	}
  
	$paragraphs = $domDocument->getElementsByTagName('p');
	if (0 === count($paragraphs)) {
		return $content;
	}

	// If so, iterate through the elements and add the post ID as a custom attribute.
	$updatedContent = '';
	foreach ($paragraphs as $paragraph) {
		$paragraph->setAttribute('data-id', get_the_ID());
	}
  
	return wp_kses_post($domDocument->saveHTML());
}