Update to work with libxml 2.9.0 

https://github.com/php/php-src/commit/c4b26cc1b0b0521c75e653fffec2a9e3b4bf8cbb

diff --git a/ext/dom/documenttype.c b/ext/dom/documenttype.c

index d61ba79..eee3b5f 100644

--- a/ext/dom/documenttype.c

+++ b/ext/dom/documenttype.c

@@ -205,7 +205,11 @@ int dom_documenttype_internal_subset_read(dom_object *obj, zval **retval TSRMLS_

 		if (buff != NULL) {

 			xmlNodeDumpOutput (buff, NULL, (xmlNodePtr) intsubset, 0, 0, NULL);

 			xmlOutputBufferFlush(buff);

+#ifdef LIBXML2_NEW_BUFFER

+			ZVAL_STRINGL(*retval, xmlOutputBufferGetContent(buff), xmlOutputBufferGetSize(buff), 1);

+#else

 			ZVAL_STRINGL(*retval, buff->buffer->content, buff->buffer->use, 1);

+#endif

 			(void)xmlOutputBufferClose(buff);

 			return SUCCESS;

 		}

diff --git a/ext/dom/node.c b/ext/dom/node.c

index 5bcb234..727d1bc 100644

--- a/ext/dom/node.c

+++ b/ext/dom/node.c

@@ -1895,9 +1895,17 @@ static void dom_canonicalization(INTERNAL_FUNCTION_PARAMETERS, int mode) /* {{{

         RETVAL_FALSE;

     } else {

 		if (mode == 0) {

+#ifdef LIBXML2_NEW_BUFFER

+			ret = xmlOutputBufferGetSize(buf);

+#else

 			ret = buf->buffer->use;

+#endif

 			if (ret > 0) {

+#ifdef LIBXML2_NEW_BUFFER

+				RETVAL_STRINGL((char *) xmlOutputBufferGetContent(buf), ret, 1);

+#else

 				RETVAL_STRINGL((char *) buf->buffer->content, ret, 1);

+#endif

 			} else {

 				RETVAL_EMPTY_STRING();

 			}

diff --git a/ext/simplexml/simplexml.c b/ext/simplexml/simplexml.c

index a379111..2368596 100644

--- a/ext/simplexml/simplexml.c

+++ b/ext/simplexml/simplexml.c

@@ -1387,7 +1387,11 @@ static int sxe_objects_compare(zval *object1, zval *object2 TSRMLS_DC) /* {{{ */

 			xmlNodeDumpOutput(outbuf, (xmlDocPtr) sxe->document->ptr, node, 0, 0, ((xmlDocPtr) sxe->document->ptr)->encoding);

 			xmlOutputBufferFlush(outbuf);

+#ifdef LIBXML2_NEW_BUFFER

+			RETVAL_STRINGL((char *)xmlOutputBufferGetContent(outbuf), xmlOutputBufferGetSize(outbuf), 1);

+#else

 			RETVAL_STRINGL((char *)outbuf->buffer->content, outbuf->buffer->use, 1);

+#endif

 			xmlOutputBufferClose(outbuf);

 		}

 	} else {