_shutdown_handler() getting called

_shutdown_handler() is always called as the framework completes running. It is part of the normal execution flow.
Look at line 140 in CodeIgniter.php where PHP is told to use _shutdown_handler().

PHP Code:

register_shutdown_function('_shutdown_handler'); 


Read about register_shutdown_function() HERE.

The shutdown handler will always be called when script execution ends, whether the end is graceful or not.

I don't think error_get_last() will return anything if the problem is an uncaught exception. Got any log or error messages about those?

Severity=8192 means that an E_DEPRECATED error was issued, which is just a warning, not fatal. Execution will continue. That tends to indicate that mbstring.internal_encoding is not the issue.