Just noticed I didn't specify the PHP versions. Testing was done on PHP 8.3.27 and PHP 8.4.14, but I went back through older versions and PHP 8.3.19 and PHP 8.4.5 were fine, crashing started happening from 8.3.20/8.4.6 onwards.
The crash is happening after loading an XML file with SimpleXMLElement and traversing/reading various child nodes from it and assigning them to an associative array.
Sorry for not being able to provide a reproducible bit of PHP code, have tried for several hours but no luck. Hopefully with the debug build there is a way to still provide enough information to find the issue.

I just had a look at the traces, and these both indicate that the undefined read happens when PHP's GC is cleaning things up. However, it doesn't tell me which thing, only that it was allocated in SimpleXMLElement_getNamespaces, and being freed through a foreach loop ending (in the PHP 8.4 case). But I can't see what, or how, which makes it impossible for me to find this. From this trace, I am not even sure if it's solely an Xdebug issue or whether SimpleXML is broken.

Could I attempt to debug it with your whole suite locally?

Thanks for the reply! Unfortunately I can't share the whole suite. The problem is that this is a integration/functional test on some legacy code with a bunch of inter-dependencies, so it doesn't work unless you have the whole application running.
But I did have some luck yesterday in reproducing it in a more isolated way with just running a single test class (which contains over 30 individual tests).
This gives some hope in that I may be able to pull out the relevant parts of the code and create something standalone that can reproduce this. Next week I will spend some time to try that.
A very weird thing happened though, I was able to consistently reproduce it by running PHPUnit for the single test case, unless I added --testdox, when I did that for some more output it stopped crashing. I have no clue how --testdox can affect whether the test crashes or not.

I would recommend running everything through valgrind, with the environment variable USE_ZEND_ALLOC=0 — turning off the Zend memory manager gives tools, such as valgrind (or the memory sanitizer) a better stab at detecting problems, as the Zend MM often hides the real point in time where an error is created. You say that you set it — but make sure to export as tools (such PHPUnit in test isolation mode) might start a new PHP without the existing environment variables. Adding --test-dox might just subtly change where the memory gets misused, and it vaguely hints to me that the USE_ZEND_ALLOC env var isn't passed on.

I am running everything in a Docker container and added ''ENV USE_ZEND_ALLOC=0'' in the ''Dockerfile'', but I will double check to make sure it's set when I start debugging again and will take a look if I can get valgrind working. Thanks for the pointers!

Hi, sorry that it took a bit, things got busy, but today I had some time to dive into this again.
However, I couldn't reproduce it anymore. Not sure why, also on 8.4.14 where it previously didn't work I failed to reproduce it.
Some other dependencies were updated in the mean time, so it's hard to say where the difference is between then and now.

This ticket can be closed, if it comes back I'll investigate more and re-open or create a new ticket.

Ok, thanks for letting us know.

And if this indeed returns, feel free to file a new ticket.