loadHTMLFile
requires path of the html file not the content of the html file loadhtmlfile. So your code will be
$doc = new DOMDocument();
$doc->loadHTMLFile("path to html file");
$xpath = new DOMXpath($doc);
Edit
If you want to load from html content use loadhtml
$doc = new DOMDocument();
$doc->loadHTML($store);
$xpath = new DOMXpath($doc);