如何使用XPath检索子子节点？

I'm testing an XML structure:

<Articles>


...


   <Article ID="333">

     <author>Paul</author>

     <title>i hate xpath </title>

        <child1>bla</child1>

       <child2>blabla
          <subchild>
             <subsubchild DEEP = "Attribute"></subsubchild>     //this is my nightmare
          </subchild>
       </child2>

       <child3>testing</child3>

   </Article> 
...

</Articles>

What XPath expression will fetch and update the subsubchild attrib value? I am mostly concerned with fetching and updating sub values of any node, no matter how deeply or superficially located, if the parent ID (333 in this case) is known.

Currently I have used :

$query= "//*[@ID=333]//*node()[@DEEP and @DEEP = "Attribute"]";
$outcome = $xml->xpath($query);
    echo isset($outcome[0][0]) ? "found" : "unavailable";

and several more related expressions, they're all frustrating my efforts with Invalid expression report.

function XMLChangeAttribValue($ParentID, $attirbName, $attribValue, $NewValue){
        
            $xml    //Never mind, i've create this sucessfully 

 
            $query = "//*[@ID=$ParentID]//*[@$attirbName = $attribValue]";
                    

            $outcome = $xml->xpath($query);
            echo isset($outcome[0][0]) ? "found" : "unavailable";
}

</div>

The XPath to select the subsubchild you are targeting in this case is:

$query = "//*[@ID=333]//*[@DEEP = 'Attribute']";

To select the attribute itself, you could use:

$query = "//*[@ID=333]//*/@DEEP[. = 'Attribute']";

Note that I used single quotes inside the string because you can't use double quotes inside a double-quoted string unless you escape them.

It should also be noted that the use of *node() all in one clump is not valid in an XPath expression.