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