The constants below are defined by this extension, and will only be available when the extension has either been compiled into PHP or dynamically loaded at runtime.
LIBXML_BIGLINES
     (integer)
   Note:
Only available as of PHP 7.0.0 with Libxml >= 2.9.0
LIBXML_COMPACT
     (integer)
   Note:
Only available in Libxml >= 2.6.21
LIBXML_DTDATTR
     (integer)
   LIBXML_DTDLOAD
     (integer)
   LIBXML_DTDVALID
     (integer)
   LIBXML_HTML_NOIMPLIED
     (integer)
   Note:
Only available in Libxml >= 2.7.7 (as of PHP >= 5.4.0)
LIBXML_HTML_NODEFDTD
     (integer)
   Note:
Only available in Libxml >= 2.7.8 (as of PHP >= 5.4.0)
LIBXML_NOBLANKS
     (integer)
   LIBXML_NOCDATA
     (integer)
   LIBXML_NOEMPTYTAG
     (integer)
   Note:
This option is currently just available in the DOMDocument::save and DOMDocument::saveXML functions.
LIBXML_NOENT
     (integer)
   LIBXML_NOERROR
     (integer)
   LIBXML_NONET
     (integer)
   LIBXML_NOWARNING
     (integer)
   LIBXML_NOXMLDECL
     (integer)
   Note:
Only available in Libxml >= 2.6.21
LIBXML_NSCLEAN
     (integer)
   LIBXML_PARSEHUGE
     (integer)
   Note:
Only available in Libxml >= 2.7.0 (as of PHP >= 5.3.2 and PHP >= 5.2.12)
LIBXML_PEDANTIC
     (integer)
   Note:
Available as of PHP >= 5.4.0
LIBXML_XINCLUDE
     (integer)
   LIBXML_ERR_ERROR
     (integer)
   LIBXML_ERR_FATAL
     (integer)
   LIBXML_ERR_NONE
     (integer)
   LIBXML_ERR_WARNING
     (integer)
   LIBXML_VERSION
     (integer)
   LIBXML_DOTTED_VERSION
     (string)
   LIBXML_SCHEMA_CREATE
     (integer)
   Note:
Only available in Libxml >= 2.6.14 (as of PHP >= 5.5.2)