You need to use the DOMDocument class, and, more specifically, its loadHTML method, to load your HTML string to a DOM object.
For example :
$string = <<<HTML
<p>test</p>
<div class="someclass">text</div>
<p>another</p>
HTML;
$dom = new DOMDocument();
$dom->loadHTML($string);
After that, you'll be able to manipulate the DOM, using for instance the DOMXPath class to do XPath queries on it.
For example, in your case, you could use something based on this portion of code :
$xpath = new DOMXpath($dom);
$result = $xpath->query('//div[@class="someclass"]');
if ($result->length > 0) {
var_dump($result->item(0)->nodeValue);
}
Which, here, would get you the following output :
string 'text' (length=4)
As an alternative, instead of DOMDocument, you could also use simplexml_load_string and SimpleXMLElement::xpath -- but for complex manipulations, I generally prefer using DOMDocument.
