<?php

$html = '<div id="container">
    <div id="test"></div>
    <pre>
        content
    </pre>
</div>';

$dom = new DOMDocument;
$dom->loadXML($html);

$xpath = new DOMXPath($dom);
$query = '//div[@id="container"]/pre';
$entries = $xpath->query($query);

foreach($entries as $one){
    $newelement = $dom->createTextNode('Some new node!'); 
    $one->parentNode->replaceChild($newelement, $one);
}

echo $dom->saveHTML();

<div id="container">
    <div id="test"></div>
    Some new node!
</div>