First, the code: 

$dom = new DomDocument();

$dom->load($url);

$root = $dom->documentElement;

Now, you can do anything you want with the $root that represents the document - use getElementsByTagName or get children using $root->childNodes.

If you try using  DOMDocument->load() statically, you will get the following error:

Non-static method DOMDocument::load() should not be called statically.

To avoid the error, you need to create an instance of the DomDocument object using the example above.