Used to trigger a user error condition, it can be used by in conjunction with the built-in error handler, or with a user defined function that has been set as the new error handler (set_error_handler()). It only works with the E_USER family of constants, and will default to E_USER_NOTICE.
This function is useful when you need to generate a particular response to an exception at runtime. For example:
Note: See set_error_handler() for a more extensive example.