OASIS Universal Business Language Schema Validator - SUNAT.
Via Composer from packagist.org.
composer require greenter/ubl-validator
use Greenter\Ubl\SchemaValidator;
$validator = new SchemaValidator();
$validator->setVersion('2.0');
$xml = file_get_contents('20000000001-01-F001-1.xml');
$result = $validator->validate($xml);
if ($result) {
echo 'Validación exitosa';
} else {
echo $validator->getMessage();
}