The simple answer is to use xpath:
$dom = new DomDocument();
$dom->loadHtml($html);
$xpath = new DomXpath($dom);
$div = $xpath->query('//*[@class="foo"]')->item(0);
But that won't accept spaces. So to select by space separated class, use this query:
//*[contains(concat(' ', normalize-space(@class), ' '), ' class ')
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