Warning: DOMDocument::importNode() ID already defined

Yeah I had also looked at the changelog, wouldn't expect such a change in a minor version update. I'm trying to find a good fix for it. It is simple to suppress these warnings with '@', but that doesn't seem like the right solution.

I feel like this is either a bug or a design flaw in libxml2. It seems to me that DOMNode::replaceChild() is now basically broken. It has now become impossible to replace a node with new content containing the same HTML IDs. You would think it should be possible to first delete the offending DOM element and then reinsert it again but both available methods DOMNode::appendChild() and DOMNode::insertBefore() are not guaranteeing that the element is inserted at the right spot: it's going to be inserted as either the first or the last sibling in the list.

I'm still having the problem with the new PHP 5.6.4 on Arch Linux.

I tried working around the problem by replacing the existing element with a temporary element with a random ID, and then replacing that again with the new element, but it resulted in a lot of very ugly code. I initially didn't want to do this but I now think it is better to simply suppress the warning until it gets sorted out in libxml2. This is acceptable since we are already suppressing similar warnings when loading HTML in DomDocument:

protected function drupalProcessAjaxResponse($content, array $ajax_response, array $ajax_settings, array $drupal_settings) {
  // ...
  // DOM can load HTML soup. But, HTML soup can throw warnings, suppress
  // them.
  $dom = new \DOMDocument();
  @$dom->loadHTML($content);

@YesCT, thinking about it again you could argue both ways. DOMDocument was already throwing warnings for duplicate IDs when loading HTML, now it also does so when importing DOMNodes into an existing document. At this point the DOMDocument is actually invalid. It doesn't care that we are importing this DOMNode so that we can later replace another node that happens to have the same ID. It cannot know our intent, so I can understand that it throws a warning. This does mean that you cannot replace any DOMNode containing an ID without encountering a warning, which is very inconvenient for us.

@joachim, this suppression with @ is used multiple times in WebTestBase already when interfacing with DomDocument, but I do agree that an explanation wouldn't hurt. Added it in the patch. I leave it on RTBC since this only adds some documentation.

@Gregory Boddin just told me that there is another way to suppress warnings in libxml: libxml_use_internal_errors(). This has been added in PHP 5.1 so that people no longer need to suppress libxml errors with @.

+++ b/.htaccess
@@ -19,6 +19,10 @@ ErrorDocument 404 /index.php
+# Add correct encoding for SVGZ.
+AddType image/svg+xml svg svgz
+AddEncoding gzip svgz
+

This is an unrelated change.

Looking good now, thanks a lot!

I wrote the original patch, but since this has already been committed for D8 and I was not involved in the backport I think it's OK for me to RTBC this.