PHP DOMDocument：从id获取属性值

I would like to extract the value of the attribute "value" using the id tag.

My code:

<?php
 $url = 'http://turni.tt-contact.com/Default.aspx';
 $contents = htmlentities(file_get_contents($url));
 echo $contents."
"; //html
 $dom = new DOMDocument;
 $dom->validateOnParse = true;
 $dom->loadHTML($contents);
 $dom->preserveWhiteSpace = false;
 $data = $dom->getElementById("__VIEWSTATE");
 echo $data->nodeValue;
?>

I would like the attribute "value" -> "THIS":

   <input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="THIS">

but the code returns only the html code.

What do I need to change?

Also by modifying it to:

$xpath = new DOMXpath($dom);
$data = $xpath->query('//input[@id="__VIEWSTATE"]');
$node = $data->item(0);
echo $node->getAttribute('value');

I get this error:

  Fatal error: Call to a member function getAttribute() on null

Try this :

$data->getAttribute('value');

PHP: DomElement->getAttribute

$attrs = array();
 for ($i = 0; $i < $data->attributes->length; ++$i){
   $node = $data->attributes->item($i);
   $attrs[$node->nodeName] = $node->nodeValue;
 }
 var_dump($attrs);

Don't use htmlentities as it will change the document's HTML tags from : <html> to &lt;html&gt;and your document won't be HTML anymore, just a plain text full of &lt; and &gt;, and so the methods to get nodes won't work.