使用PHP解析<meta itemprop =“a”content =“b”>（不在HEAD中）

I need to parse some html code that looks like this:

<div id="item-1" class="genclass" itemscope itemtype="http://schema.org/Book">
    <meta itemprop="isbn" content="XXXXXXXXXXXX" />
    <meta itemprop="name" content="Neverending Story" />
    <meta itemprop="author" content="Michael Ender" />
    <meta itemprop="publisher" content="MyBooks" />
    <meta itemprop="datePublished" content="1991" />
    <h2 itemprop="offers" itemscope  itemtype="http://schema.org/Offer">
        <meta itemprop="price" content="6.6" />
        <meta itemprop="priceCurrency" content="USD" />
    </h2>
</div>

So I'm trying:

libxml_use_internal_errors(TRUE);
$dom->loadHTMLFile($url);
libxml_clear_errors();
foreach($dom->find("meta[itemprop='isbn']") as $books){
    switch ($books->itemprop) {
        case 'isbn':
        $line['isbn'] = $books->content;
        break;
    case 'name':
        $line['name'] =  $books->content;
        break;
    case 'author':
        $line['author'] =  $books->content;
        break;
    case 'publisher':
        $line['publisher'] =  $books->content;
        break;
    case 'datePublished':
        $line['datePublished'] =  $books->content;
        break;
    case 'price':
        $line['price'] =  $books->content;
        break;
    default:
        break;
    }
}
print_r($books);

But the result is always blank. What am I doing wrong? I've tried with get_meta_tags and others...

</div>