Search particular node value from xml

This is my php/xml code. I just want to search that particular $searchterm from this :

$doc = new DOMDocument;
$doc->load('authentication.xml');

$xpath = new DOMXPath($doc);

$searchTerm = "N0A097016646";

$lineNo = 0;
foreach ($xpath->query('//device/following::comment()') as $comment){
    $serial= $xpath->query('//device', $comment)->item($lineNo)->textContent;

    echo "Line number: ".$comment->getLineNo()." Device number: ".$serial." Comented number: ".$comment->textContent."<br>";

    $lineNo++;
}

To where I should put $searchTerm varible to search that particular node? Please help.