How can I do this without Docker? Your docker container/setup likely has no debugging set-up whatsover...

You can do this by installing php-fpm (I tested on 7.4.3 and 7.4.4), the ddtrace extension (I used 0.42.0, install instructions: https://docs.datadoghq.com/tracing/setup/php/, github repo: https://github.com/DataDog/dd-trace-php) and xdebug.
Please make sure that the extension=ddtrace line is before zend_extension=xdebug in php.ini.

ddtrace setings
ddtrace.request_init_hook= - a valid path to a PHP script (the contents don't seem to matter).
ddtrace.disable=Off

xdebug settings:
xdebug.remote_enable=1

Did you actually check this with Xdebug 2.9.4? Because this is the same issue as 0001736, which was fixed by the authors of the ddtrace extension with the exact log message:

Fixed issue #1736: Segmentation fault when other extensions run PHP in RINIT

Other extensions that run PHP in RINIT before Xdebug's RINIT (when XG_DBG(breakable_lines_map) is allocated) will cause a segmentation fault.

The fix for that is part of Xdebug 2.9.2.

Yes. It's still failing on 2.9.2, 2.9.3 and 2.9.4. The interesting detail is that it's segfaulting on all of the requests on 2.9.1, but only on around half of them on 2.9.2 and later.

I think the issue is that breakable_lines_map isn't initialized here: https://github.com/xdebug/xdebug/blob/2afa19428cada525b5cc2f01752f1036c358adef/src/debugger/debugger.c#L51 so there's no guarantee that the pointer will be NULL before the hashmap is allocated

I think it's just missing a

XG_DBG(breakable_lines_map) = NULL;

below:

https://github.com/xdebug/xdebug/blob/2afa19428cada525b5cc2f01752f1036c358adef/src/debugger/debugger.c#L644

But I want to reproduce it first :-)

This is now fixed in GIT. I could reproduce this with "php -S" and two requests.

That's wonderful news. Thank you!

Thanks Derick! I can confirm that this commit fixes the issue on my end as well. <3

https://github.com/xdebug/xdebug/commit/6c6c08233593ffc1d64d70c51c56f567e6528010