RegEx & PHP: last appearance with $ , only working in online tester - php

Question

So I got the following HTML Code:
[et_pb_section admin_label="section"]
<h2>abcdefghijkl - abcdefghijklmno</h2>
<p> </p>
<p>abcdefghi<strong> abcdefghijkl</strong> abcdefghijklmnopqrstuvxyz.</p>
<p><br /><br /><span style="text-decoration: underline;"><strong>abcdefghij</strong></span></p>
<p><br />abcdefghijklmnopqrstuvxyz.<br /> <br /><br />
<span style="text-decoration: underline;"><strong>abcdefghi</strong>
</span></p>
<p><br />abcdefghijklmano</p>
<br /><br />
<div id="termine">
<div>
<strong>abcdefghikla - 12345678</strong>
<p>
<a href="/just-another-url.html">abcdefghijklmnopqrst <br />
abcdefghijklmnopqrst >>></a>
</p>
</div>
</div>
</div>
and I want to replace the last closing </div> with another piece of code [/et_pb_section].
So I have tried in one of the many online regex testers and cameup with this
$content= preg_replace("/<\/div>$/", $et_pb_section_ENDTAG, $content); where $et_pb_section_ENDTAG is
$et_pb_section_ENDTAG='[/et_pb_section]';.
When using the online tester everything works fine and the last </div> gets replaced but inside my php script it is not working. nothing happens, no error, no nothing. The HTML code stays the same. What am I doing wrong here?
Thank you.
EDIT: Oh, I almost forgot, when I get rid of the $ and use RegEx Option D ( matches only at the end of string) then all three closing </div> get replaced. So I guess something is wrong with the $

Answer 1

An example with DOMDocument, DOMXPath and DOMDocumentFragment that replaces the first div tag with an "admin_label" attribute with the value "section" (feel free to adapt to your real needs):
$html = <<<'EOD'
<div admin_label="section">
<p>abdefghij</p>
<p>klmnopqrs</p>
<p>tuvwxyz01</p>
<ul>
<li>2345</li>
<li>6789</li>
</ul>
</div>
EOD;
libxml_use_internal_errors(true);
$dom = new DOMDocument;
$dom->loadHTML($html, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);
$xp = new DOMXPath($dom);
$divNode = $xp->query('(//div[#admin_label="section"])[1]')->item(0);
$open = $dom->createTextNode('[et_pb_section admin_label="section"]');
$close = $dom->createTextNode('[/et_pb_section]');
$fragment = $dom->createDocumentFragment();
$fragment->appendChild($open);
foreach ($divNode->childNodes as $childNode) {
$fragment->appendChild($childNode->cloneNode(true));
}
$fragment->appendChild($close);
$divNode->parentNode->replaceChild($fragment, $divNode);
echo $dom->saveHTML();
libxml_clear_errors();
Note: in real life, you need to check if the XPath query returns something before continue.