shutdown callback is not invoked when calling undefined method

Question

Consider the following two PHP (5.4) scripts. Why is the callback passed to register_shutdown_function only invoked when script A is executed, but not when script B is executed?

Script A

set_error_handler(function() {
    throw new Exception();
});
register_shutdown_function(function() {
    echo "shutdown handler invoked\n";
});

undefined();
// "shutdown handler invoked" IS displayed

Script B

set_error_handler(function() {
    throw new Exception();
});
register_shutdown_function(function() {
    echo "shutdown handler invoked\n";
});

$undefined->undefined();
// "shutdown handler invoked" IS NOT displayed

Answer 1

It's a bug—if the callable registered with set_error_handler throws an exception, the shutdown function will not be invoked.

In this particular case, the following chain of events happens:

1. Non-fatal error is triggered (Undefined variable: undefined)
2. User error handler is invoked
3. Exception is thrown
4. Fatal error is triggered (Call to a member function undefined() on a non-object)
5. Shutdown function is not invoked, due to existing exception

The existing bug reports at https://bugs.php.net/61767 (with patch!) and https://bugs.php.net/60909 have additional details.

Answer 2

Here is what I found out.

register_shutdown_function is called upon exit.

$undefined->undefined(); triggers E_NOTICE error because $undefined is undefined as a variable which goes through set_error_handler. if you call throw new Exception, php for some reason doesn't trigger register_shutdown_function handler when exiting just crashes, if you want to throw an exception maybe you can call your exception function directly.

If you make a dummy class and define $undefined, but doesn't make a member function called undefined, it will be behave in the same way as calling undefined() as below

undefined(); triggers E_ERROR which immediately exits triggering shutdown_function handler

You could do this in the shutdown function beside using error handler to catch different types of errors:

function shutdown_function_handler(){
    if ($error = error_get_last()){
        switch ($error['type']){
            case E_ERROR:
            case E_USER_ERROR:
            case E_CORE_ERROR:
            case E_COMPILE_ERROR:
                echo " ERRROR:".$error['message'];
        }
    }
}