$html = '<div class="embed-responsive embed-responsive-16by9">
  <iframe class="embed-responsive-item" src="http://www.google.com"></iframe>
</div>';

$dom = new domDocument;
$dom->loadHTML($html);
$dom->preserveWhiteSpace = false;

$finder = new DomXPath($dom);
$classname="embed-responsive-item";
$nodes = $finder->query("//*[contains(concat(' ', normalize-space(@class), ' '), ' $classname ')]");

for ($x = 0; $x < $nodes->item(0)->attributes->length; $x++){
    If ($nodes->item(0)->attributes->item($x)->name == "src"){
        $src = $nodes->item(0)->attributes->item($x)->value;
    }
}

//Will return "http://www.google.com"
echo $src;