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php fatal error handler… failing with no error_get_last() set

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I have been using a fatal_handler() function that I expanded on for over a year now but for some reason I now have random errors popping up in the error_log file on the server and triggering the notification. Previous to a little while ago it was working fine.

The errors seem to come from the array $error which is suppose to be set by error_get_last(). With that, I am even unsure why the error event is firing. Any of my logs that the function creates are empty. It doesn’t give me any specific page causing the error, just the function itself.

The $notice->rMsg saves the notification in a session variable and then displays anything in that session variable the next page load. Sometime I have 10 or 20 of these notifications popping up when I shouldn’t. The page still loads, I am not redirected to the system logs, and I can continue on but the errors still keep coming. I do not know what is wrong but hopefully someone can point me in the right direction to look.

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LOG FILE

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UPDATE: January 9, 2022 11:40am

It appears the script below is causing the “error” and the event to fire. This is for an administration system which if the user is not active it places a lock screen after so many minutes requiring a password again. This script keeps the session active so the lock screen continues to work for a couple hours instead of loosing the session and requiring a full login again. If the browser resets the session it will redirect back to the main login screen.

This is called via ajax every couple minutes or so; which is why when I reload the page I get multiple error notices produced by the fatal_handler(). However, I don’t know why php is registering a shutdown as the error_get_last() is still blank.

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Answer

As the name suggests, register_shutdown_function registers a handler to run every time PHP “shuts down” – that is, at the end of every request, regardless of whether it’s ending “normally” or after an error.

It’s therefore up to you, inside the handler function, to detect whether the shutdown was due to a fatal error or not. The common trick with this is to look at the last “error”, which might better be termed “diagnostic”, because it includes Warnings, Notices, and Deprecation messages. If there was a fatal error, we can expect it to be the last thing that happened before the shutdown handler is called, so if we immediately call error_get_last, we should get the information about it.

Let’s trace through some key parts of your function:

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Luckily, the default of E_CORE_ERROR and the broken check for NULL cancel each other out, so that if there was no error returned from error_get_last(), we are checking in_array(NULL,$log_errors), which will be false.

However, looking at $log_errors, which is defined as a global variable earlier:

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Apart from the slightly odd decision to hard-code the numbers rather than use the actual constants, note that this includes both Warnings and Notices. These can happen at any time, and there might have been a dozen Notices already logged on the page, but error_get_last() will just contain the most recent if no other diagnostics happened afterwards. It’s therefore not relevant to look for these inside the shutdown handler – you just want to spot things that would cause a shutdown.

A Notice or Warning will not cause a script to exit early, so it makes no sense to log them in a shutdown handler, which will only see whichever happened to come last. They will already have been logged based on your error_log settings, or you can use set_error_handler to have custom handling of all of them.

We can massively simplify the function by checking for fatal errors (and only fatal errors) straight away, and just ending the function if none are found:

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