While tinkering for an answer to this question, I found that debug_backtrace() doesn't trace beyond the function registered to register_shutdown_function(), when called from within it.
This was mentioned in this comment for register_shutdown_function() in the PHP docs, stating:
You may get the idea to call debug_backtrace or debug_print_backtrace from inside a shutdown function, to trace where a fatal error occurred. Unfortunately, these functions will not work inside a shutdown function.
Explained with a bit more detail, comments on this answer state:
Doesn't work. The shutdown function occurs after the stack has unwinded. There is no stack information to dump.
Is there any way to circumvent this, forcing PHP to hold the stack trace until the process has terminated altogether, or should we accept it as a given due to PHP internals?
Is there any way to circumvent this, forcing PHP to hold the stack trace
That's rather meaningless, when the registered function is invoked, all your defined functions have returned or been cleared down from the stack.
If you need to know where your code exited, then you need to instrument your code.