<?php

$html = <<<HTML
<p>...</p>

<div id="div1">
	<div id="div1-1">...</div>
	<div id="div1-2">...</div>
	...
</div>

<p>...</p>

<div id="div2">...</div>
HTML;

$doc = new DOMDocument();
$doc->loadHTML($html);

$xpath = new DOMXPath($doc);
$divs = $xpath->query('//div[not(ancestor::div)]');

foreach($divs as $div) {
	echo $div->getAttribute('id'), PHP_EOL;
}