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 {