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Fixes GHSA-3qrf-m4j2-pcrr. To parse a document with libxml2, you first need to create a parsing context. The parsing context contains parsing options (e.g. XML_NOENT to substitute entities) that the application (in this case PHP) can set. Unfortunately, libxml2 also supports providing default set options. For example, if you call xmlSubstituteEntitiesDefault(1) then the XML_NOENT option will be added to the parsing options every time you create a parsing context **even if the application never requested XML_NOENT**. Third party extensions can override these globals, in particular the substitute entity global. This causes entity substitution to be unexpectedly active. Fix it by setting the parsing options to a sane known value. For API calls that depend on global state we introduce PHP_LIBXML_SANITIZE_GLOBALS() and PHP_LIBXML_RESTORE_GLOBALS(). For other APIs that work directly with a context we introduce php_libxml_sanitize_parse_ctxt_options().