how to parse an input value thats hidden

I can't find anything on here or google related to parsing input values that are hidden. For example this piece of code here. I am trying to parse the 40 character key.

<input type="hidden" name="key" value="c126b4f134cb2c1184c1585fdfa4d1b0013a12f4"> 

i tried this but it never returns the value of anything hidden.

libxml_use_internal_errors(TRUE);
    $dom = new DOMDocument;
    $dom->loadHTMLFile('http://www6.cbox.ws/box/?boxid=524970&boxtag=7xpsk7&sec=form');
    libxml_clear_errors();

    $xp = new DOMXpath($dom);
    $nodes = $xp->query('//input/@value');
    foreach($nodes as $node)
        {
          echo( $node->textContent . "<br><br>" );
        }

var_dump($node);

update code

libxml_use_internal_errors(TRUE);
$dom = new DOMDocument;
$dom->loadHTMLFile('http://www6.cbox.ws/box/?boxid=524970&boxtag=7xpsk7&sec=form');
libxml_clear_errors();

$xp = new DOMXpath($dom);
$nodes = $xp->query('//input[type="hidden"]');
$val = $nodes->getAttribute('value');

v开发者_开发知识库ar_dump($val);

returns this error referring to line "$val = $nodes->getAttribute('value');"

Fatal error: Call to undefined method DOMNodeList::getAttribute()

$nodes = $xp->query('//input[type="hidden"]');
foreach ($nodes as $node) {
    $val = $node->getAttribute('value');
}

comment followup:

If you look at the source of the page you've included in your code sample, you'll see there's only a single hidden form field, and it's got an empty value:

<input type="hidden" name="key" value="">

so of course the XPath will return a NULL - that's what's stored in that value attribute: nothing.

Using your update code, the fixed version would be:

<?php

libxml_use_internal_errors(TRUE);
$dom = new DOMDocument;
// This link is now dead...
$dom->loadHTMLFile('http://www6.cbox.ws/box/?boxid=524970&boxtag=7xpsk7&sec=form');
libxml_clear_errors();

$val = array(); // Must be set before foreach loop

$xp = new DOMXpath($dom);
$nodes = $xp->query('//input[type="hidden"]');
foreach ($nodes as $node) {
    $val[] = $node->getAttribute('value');
}

var_dump($val);

?>