If you wonder, what values those constant have, here you go:
<?php
XML_ERROR_NONE = 0
XML_ERROR_NO_MEMORY = 1
XML_ERROR_SYNTAX = 2
XML_ERROR_NO_ELEMENTS = 3
XML_ERROR_INVALID_TOKEN = 4
XML_ERROR_UNCLOSED_TOKEN = 5
XML_ERROR_PARTIAL_CHAR = 6
XML_ERROR_TAG_MISMATCH = 7
XML_ERROR_DUPLICATE_ATTRIBUTE = 8
XML_ERROR_JUNK_AFTER_DOC_ELEMENT = 9
XML_ERROR_PARAM_ENTITY_REF = 10
XML_ERROR_UNDEFINED_ENTITY = 11
XML_ERROR_RECURSIVE_ENTITY_REF = 12
XML_ERROR_ASYNC_ENTITY = 13
XML_ERROR_BAD_CHAR_REF = 14
XML_ERROR_BINARY_ENTITY_REF = 15
XML_ERROR_ATTRIBUTE_EXTERNAL_ENTITY_REF = 16
XML_ERROR_MISPLACED_XML_PI = 17
XML_ERROR_UNKNOWN_ENCODING = 18
XML_ERROR_INCORRECT_ENCODING = 19
XML_ERROR_UNCLOSED_CDATA_SECTION = 20
XML_ERROR_EXTERNAL_ENTITY_HANDLING = 21
?>