<?php
$file = __DIR__ . '/test.xml';
$dom = new DOMDocument('1.0');
$dom->load($file);
$xpath = new DOMXPath($dom);
// regular xpath expression
$title = $xpath->query('/catalog/book[4]/title');
echo $title->item(0)->nodeValue . PHP_EOL;
// register a php function
$xpath->registerNamespace("php", "http://php.net/xpath");
$xpath->registerPHPFunctions(['strlen']);
// use php function in xpath expression
$title = $xpath->query('/catalog/book[php:functionString("strlen", title) = "15"]/title');
echo $title->item(0)->nodeValue . PHP_EOL;