Fix DOMDocument UTF-16 parsing

We should be using xmlCreateMemoryParserCtxt for in-memory XML documents because it correctly handles UTF-16 encoded strings.

hphp/runtime/ext/ext_domdocument.cpp

@@ -653,7 +653,8 @@ static String _dom_get_valid_file_path(const char *source) {
 }
 
 static xmlDocPtr dom_document_parser(c_DOMDocument * domdoc, int mode,
-                                     char *source, int options) {
+                                     char *source, int source_len,
+                                     int options) {
   xmlDocPtr ret = NULL;
   xmlParserCtxtPtr ctxt = NULL;
 
@@ -672,7 +673,7 @@ static xmlDocPtr dom_document_parser(c_DOMDocument * domdoc, int mode,
       ctxt = xmlCreateFileParserCtxt(file_dest.data());
     }
   } else {
-    ctxt = xmlCreateDocParserCtxt((xmlChar*)source);
+    ctxt = xmlCreateMemoryParserCtxt(source, source_len);
   }
 
   if (ctxt == NULL) {
@@ -752,7 +753,8 @@ static Variant dom_parse_document(c_DOMDocument *domdoc, CStrRef source,
     return false;
   }
   xmlDoc *newdoc =
-    dom_document_parser(domdoc, mode, (char*)source.data(), options);
+    dom_document_parser(domdoc, mode, (char*)source.data(), source.length(),
+                        options);
   if (!newdoc) {
     return false;
   }

hphp/test/zend/good/ext-xml/bug50661.php

@@ -0,0 +1,6 @@
+<?php
+$data = "\xFE\xFF\x00\x3C\x00\x66\x00\x6F\x00\x6F\x00\x2F\x00\x3E";
+
+$dom = new DOMDocument();
+$dom->loadXML($data);
+echo $dom->saveXML();

hphp/test/zend/good/ext-xml/bug50661.php.expectf

@@ -0,0 +1,2 @@
+<?xml version="1.0"?>
+<foo/>