debug_backtrace() from registered shutdown function in PHP

Question

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?

Answer 1

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.

Answer 2

This is a very expensive solution. I never used register_tick_function() or tick and I'm not sure if it works as expected.

declare(ticks=1);

function tick_handler() {
    global $backtrace;
    $backtrace = debug_backtrace();
}
register_tick_function('tick_handler');



function shutdown() {
    global $backtrace;
    // do check if $backtrace contains a fatal error...
    var_dump($backtrace);
}
register_shutdown_function('shutdown');

Answer 3

In XDebug extension, there is a xdebug_get_function_stack() function.

This one works similar to PHP's internal debug_backtrace(), but keeps the trace even in shutdown handler.

You won't get the exact exit point though, only the last executed function before the shutdown occured (triggered by die()/exit() call or error).

This is of course suitable only for development environment.

Answer 4

From my experience, the shutdown function starts with a clean stack, and it has no access to the "original" stack (as it no longer exists at that point).

Unfortunately, there is no way to save that original stack.

Answer 5

Inside your registered shutdown function you can get backtrace by error_get_last function. Works for me in PHP7.