Drupal + PHP - Markup manipulation - Weave text inside of the initial <p> tag of a text field

code.php

<?php
use Drupal\Component\Utility\Html;

// For example, we have a node with a "tracking ID" that we want to weave in as an image element to the first paragraph.
// This snippet will alter that markup so when we pass it through to whatever renderer,
// we've leveraged PHP built in XML editing to "weave" this snippet into the first <p> tag
  $description = $some_node->body->value;
  $tracking_markup = "<img src='whatever' />";
  if (!empty($description)) {
    // We might get wonky HTML, so we don't want the xml lib warnings to flood watchdog.
    libxml_use_internal_errors(TRUE);
    $html_dom = Html::load($some_node->body->value);
    // This gets the p tags, but could do any other tag here.
    $paragraphs = $html_dom->getElementsByTagName('p');
    $first_paragraph = $paragraphs->item(0);
    $first_paragraph_text_content = $first_paragraph->textContent;
    $updated_paragraph_text = $html_dom->createDocumentFragment();
    $updated_paragraph_text->appendXML('<p>' . $tracking_markup . $first_paragraph_text_content . '</p>');
    $first_paragraph->parentNode->replaceChild($updated_paragraph_text, $first_paragraph);
    $description = Html::serialize($html_dom);
    // Manipulate as needed before passing to feed.
    $description = str_replace("&#13;", "", $description);

  }