In order to get all meta tags you should make use of XPath to select all nodes
$xmlsource = 'http://www.example.com/';
$d = new DOMDocument();
$d->loadHTML($xmlsource);
$xpath = new DOMXPath($d);
//find all elements with itemprop attribute
$nodes = $xpath->query('//*[@itemprop]');
foreach ($nodes as $node) {
}