How Can i get the child element using class using php DOMXPath?

Question

I want to get the child element with specific class form html I have manage to find the element using tag name but can't figureout how can I get the child emlement with specific class?

Here is my CODE:

    <?php

$html = file_get_contents('myfileurl'); //get the html returned from the following url

$pokemon_doc = new DOMDocument();

libxml_use_internal_errors(TRUE); //disable libxml errors

if (!empty($html)) { //if any html is actually returned
    $pokemon_doc->loadHTML($html);
    libxml_clear_errors(); //remove errors for yucky html

    $pokemon_xpath = new DOMXPath($pokemon_doc);

    //get all the h2's with an id
    $pokemon_row = $pokemon_xpath->query("//li[@class='content']");

    if ($pokemon_row->length > 0) {
        foreach ($pokemon_row as $row) {
            $title = $row->getElementsByTagName('h3');
            foreach ($title as $a) {
                echo "Title: ";
                echo strip_tags($a->nodeValue). '<br>';
            }
            $links = $row->getElementsByTagName('a');
            foreach ($links as $l) {
                echo "Link: ";
                echo strip_tags($l->nodeValue). '<br>';
            }
            $desc = $row->getElementsByTagName('span');

            //I tried that but didnt work..... iwant to get the span with class desc 
            //$desc = $row->query("//span[@class='desc']");

            foreach ($desc as $d) {
                echo "DESC: ";
                echo strip_tags($d->nodeValue) . '<br><br>';
            }
            //  echo $row->nodeValue . "<br/>";
        }
    }
}
?>

Please let me know if this is a duplicate but I cant find out or you think question is not good or not explaining well please let me know in comments.

Thanks.