`set_error_handler` should be able to handle fatal error once

[mvorisek]

Description

Currently, fatal error can be handled, but it is not possible to do so using handler set by set_error_handler, but workaround using register_shutdown_function. This is not only too complex, but there is no guarantee such handler is/was registered as the first shutdown function.

An example is here: https://github.com/phpstan/phpstan-src/blob/1.10.50/src/Command/CommandHelper.php#L137-L154

This is a feature request to allow error handler to handle fatal error when php enters into shutdown before the 1st shutdown handler is executed.

[arnaud-lb]

The issue with this is that fatal errors are triggered when the engine may be in a state in which it's not safe to execute PHP code.

One alternative is to convert fatal errors to exceptions when possible, so that the engine can return to a safe state before the exception is handled. Many fatal errors were converted some times ago, but I suspect that the remaining ones are expensive or not worth to convert.

An other alternative would be expose the backtrace of the fatal error to the shutdown function (via a function similar to error_get_last(), maybe), or other relevant information for an error handling use-case.

We could also delay handling of fatal errors until the shutdown stage, at which point it could be safer to execute user error handlers.

In general, even non-fatal errors are triggered from dangerous places, and cause all sorts of issues and complexities. It would be beneficial to delay execution of user error handlers until a safe execution point is reached, like we do for signals.

[mvorisek]

The issue with this is that fatal errors are triggered when the engine may be in a state in which it's not safe to execute PHP code.

In a state that does not allow exception to be caught (if it would be thrown), but code can be already executed thru register_shutdown_function. So this issue is about to invoke set_error_handler before the first registered function thru register_shutdown_function is executed.

[arnaud-lb]

This makes sense. If we execute shutdown functions after a fatal error, we may as well execute error handlers at the same point of the program.