replace first occurrence of "string" in nodeValue with <a> with DOMDocument

$doc = new DOMDocument();
$doc->load($file);

$xpath = new DOMXPath($doc);
foreach($xpath->query("//node()[contains(., 'string')]") as $node) {
    $node->nodeValue = preg_replace('/string/', '<a>', $node->nodeValue, 1);
}

$doc->save($file);