Apparently it isn't fatal, so I don't know what xdebug_get_function_stack() returns in this case (null maybe?)

It returns an empty array, the same return type as it would use when it can return results.

but wouldn't it be better if function_exists('xdebug_get_function_stack') was false if the function is "disabled"?

Yes, that would be better. But I can't do that unless I "hack" PHP's internal function_exists(). This is because functions have to be compiled into PHP (compile time) whereas Xdebug needs to disable these per-process (runtime) due to various variations of the xdebug.mode setting. I'm not happy with this situation either, but as it shouldn't "break" code, and these functions are meant for development purposes (like Xdebug itself), I don't want to investigate coming up with hacks to do the result that you (and I) expected. Mostly, because these hacks are invariably going to cause some other issues that then require time and effort to figure out and fix.

I just came across this piece of code again.

What would be the best way to avoid the warning? Using an @ before xdebug_get_function_stack() might work but I don't really like to use @ because when it hides a different error than the one I expected it turns into a debugging nightmare.

Is there a way to check whether the function is enabled before I call it?