Sets the default exception handler if an exception is not caught within a
try/catch block. Execution will stop after the
Name of the function to be called when an uncaught exception occurs.
This function must be defined before calling
set_exception_handler(). This handler function
needs to accept one parameter, which will be the exception object that
Returns the name of the previously defined exception handler, or NULL on error. If
no previous handler was defined, NULL is also returned.