I'm sorry I don't get it. Can you please provide a reproducer script/app where I could see the fatal error happening?

IMHO, $previousHandler can be true and then it never goes in this set_error_handler or it equals the previous error handler (null or a callable).
So there is nothing to fix.

@nicolas-grekas I don't have a case or script where it fails, just having a look at the code reveals some sort of logic error here.

@fancyweb actually you could change the line
return $previousHandler ? $previousHandler($type, $message, $file, $line) : false; simply to return false; and the 'problem' would be gone. I wanted to keep the original logic found here though, as I wasn't quite sure about the impact that the removal would have...

@tweichart ok thank for the feedback. To me, there is no bug here, things are as expected. If you can provide a script producing the fatal error you see, I'd be happy to be wrong.

@tweichart cf https://3v4l.org/vYo1c

$previousHandler in the error handler will always be equals to the previous error handler, thus null or a callable.

PHPStorm reports that it's a bool but it's a wrong static analysis.

@fancyweb yes you're right, the $previousHandler($type, $message, $file, $line) will never be called. so a more proper fix would be to replace the return statement by a simple return false;, isn't it?
If it's ok to leave unused calls in the code it's fine with me, just wanted to clean up ;-)

It can be called if there is a previous error handler (uncomment the set_error_handler(function () {});) in my previous link.

Even if it doesn't look so obvious, I am convinced that the current code works as expected. However, I have opened #32760 to rearrange some code that makes this more clear. What do you think?

Thanks @xabbuh
@tweichart please have a look at #32760 which provides the same logic as the current one, in a more readable way.