What is wrong with this code? DOMDOCUMENT

Where is the error in this code? if DOMDOCUMENT is replaced with previussibling, $ doc & nbsp; new & nbsp; DOMDocument (); $ html & nbsp; & lt; & lt; HTML & lt; html & gt; & lt; body & gt; & lt; ul & nbsp; & gt; & lt; li & gt; f. where is the error code DOMDOCUMENT?
If you change to previussibling, it will be normal.

$doc = new DOMDocument();
$html = <<
 


 
 

  
  

  
  
Foo
  
  

  
  
Bar
  
  

 
 


heading3

heading3


 
 

  
  

  
  
list2
  
  

  
  
list2
  
  

 
 




HTML;


$doc ->loadHTML($html);

$ul = $doc->getElementsByTagName('*');

foreach($ul as $node) {
     if ($node -> hasAttribute('class')) {

           foreach($node -> nextSibling ->childNodes as $morenodes) {

               print_r($morenodes);
               }
    }
  }

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------ Solution --------------------
H2's next is h3, h3 has no child
------ Solution --------------------
In this way, should we want to get all ul?

$ Ul = $ doc-> getElementsByTagName ('*');
In this way, all elements are obtained.
Change to $ ul = $ doc-> getElementsByTagName ('Ul ');
In this way, the ul of the page will be obtained ..

In this case, the following loop operation will be okay.