protected function assertXPathExpression($expected, $xmlString, $expression)
{
$dom = new \DOMDocument();
$result = @$dom->loadXml($xmlString);
if (false === $result) {
throw new \RuntimeException(sprintf('Could not load XML "%s"', $xmlString));
}
$xpath = new \DOMXPath($dom);
$dom->formatOutput = true;
$result = $xpath->evaluate($expression);
$this->assertEquals($expected, $result);
}