If the system is configured with PrivateTmp, then when Xdebug writes to "/tmp", it is actually redirected to the configured PrivateTmp directory. Because it can write there, you won't get any warnings. Unfortunately when I checked in the past to see how I can find that PrivateTmp is enabled, I found no way to do so. Otherwise I would have added such a warning already.

However, I don't think the issue here is "Profiler can't write cachegrind file", but rather that it shows up in a directory that you didn't expect. Can you confirm that?

Yes, you're correct. I expected it have to be at "/tmp" but found the cachegrind file at private temp directory like "/tmp//systemd-private-6ff598b83ece420295c55c310ace4e20-apache2.service-a8yDRu/tmp/cachegrind.out.31644". But when I invoke the function "xdebug_info()" it shows the wrong path as in the attached image. So adding extra information about this would be helpful.

Screenshot from 2022-02-21 17-41-07.png (12,358 bytes)   

Screenshot from 2022-02-21 17-41-07.png (12,358 bytes)   

I agree, but so far I have not yet found a way on how to detect this, and give you the right file name there. From the PHP process (and hence Xdebug's) point of view, the file is written to /tmp, it's just that the OS sneakily has changed what /tmp meant to some private tmp directory.

I've made a PR to warn when/if this happens:
https://github.com/xdebug/xdebug/pull/824

Fixed for Xdebug 3.2.0 (coming out in a few months). Please try GitHub's latest from the master branch, if you want to try it out!

Thanks for your help and the clarification. Will try Xdebug 3.2.0 sooner.