我在php7.2中遇到此错误。我正在使用symfony4。

Warning: Use of undefined constant XML_PI_NODE - assumed 'XML_PI_NODE' (this will throw an Error in a future version of PHP)

这是错误行。

in var/cache/dev/ContainerIZRQnYa/getSerializerService.php (line 55)
return $this->services['serializer'] = new \Symfony\Component\Serializer\Serializer([0 => new \Symfony\Component\Serializer\Normalizer\JsonSerializableNormalizer(), 1 => new \Symfony\Component\Serializer\Normalizer\DateTimeNormalizer(), 2 => new \Symfony\Component\Serializer\Normalizer\ConstraintViolationListNormalizer(), 3 => new \Symfony\Component\Serializer\Normalizer\DateIntervalNormalizer(), 4 => new \Symfony\Component\Serializer\Normalizer\DataUriNormalizer(), 5 => new \Symfony\Component\Serializer\Normalizer\ArrayDenormalizer(), 6 => new \Symfony\Component\Serializer\Normalizer\ObjectNormalizer($a, new \Symfony\Component\Serializer\NameConverter\MetadataAwareNameConverter($a), new \Symfony\Component\PropertyAccess\PropertyAccessor(false, false, new \Symfony\Component\Cache\Adapter\ArrayAdapter(0, false)), NULL, new \Symfony\Component\Serializer\Mapping\ClassDiscriminatorFromClassMetadata($a))], [0 => new \Symfony\Component\Serializer\Encoder\XmlEncoder(), 1 => new \Symfony\Component\Serializer\Encoder\JsonEncoder(), 2 => new \Symfony\Component\Serializer\Encoder\YamlEncoder(), 3 => new \Symfony\Component\Serializer\Encoder\CsvEncoder()]);

我已经清除了缓存，并且之前已经设置了所有权限。我应该怎么办？