I'm getting a new test failure in 8.3.2 and this is my prime suspect:

$ cat DOMNode_isEqualNode.diff 
--
     --- Test documents ---
     bool(true)
     bool(false)
079- bool(true)
079+ bool(false)
     bool(false)
     bool(false)
     bool(false)
--

Anything I can do to help debug it?

@orlitzky that's a libxml2 bug: https://gitlab.gnome.org/GNOME/libxml2/-/issues/634
In general, libxml2 2.12.0 and 2.12.1 are very buggy and I advice to avoid those 2 versions, libxml2 2.12.2 should be fine.

I'm getting the test failure with 2.11.15 and 2.11.16 💀

@orlitzky I just compiled and tested these libxml2 versions:
2.11.5 works
2.11.6 works
2.12.0 broken
2.12.1 broken
2.12.2 works

Specifically, this line is failing for you: var_dump($dom1->isEqualNode($dom1Clone));.
In other words: cloning a document does not create the same document.

Could you please apply the following change to the test file DOMNode_isEqualNode.phpt and report back the output?

diff --git a/ext/dom/tests/DOMNode_isEqualNode.phpt b/ext/dom/tests/DOMNode_isEqualNode.phpt
index 94031aa797..953c9a6e82 100644
--- a/ext/dom/tests/DOMNode_isEqualNode.phpt
+++ b/ext/dom/tests/DOMNode_isEqualNode.phpt
@@ -206,6 +206,7 @@ function comparePairs($list1, $list2) {
 $dom1Clone = clone $dom1;
 var_dump($dom1->documentElement->isEqualNode($dom1Clone->documentElement));
 var_dump($dom1->documentElement->isEqualNode($dom2->documentElement));
+var_dump($dom1->saveXML(), $dom1Clone->saveXML());
 var_dump($dom1->isEqualNode($dom1Clone));
 var_dump($dom1->isEqualNode($dom2));
 var_dump($dom1->documentElement->isEqualNode($dom1Clone));

Ah, crap. It was the fix for CVE-2023-45322 in libxml that is causing the regression, and we've backported it to 2.11.x. There's a newer fix in https://gitlab.gnome.org/GNOME/libxml2/-/commit/8707838e69f9c6e729c1d1d46bb3681d9e622be5. I'll ping the libxml maintainers to see if they can update the patch. Not a PHP issue. Thanks!