Identical to mktime() except the passed parameters represents a GMT date.
Parameters always represent a GMT date so is_dst doesn't influence the result.
Like mktime(), arguments may be left out in order from right to left, with any omitted arguments being set to the current corresponding GMT value.
Note: As of PHP 5.1.0, the is_dst parameter became deprecated. As a result, the new timezone handling features should be used instead.
Note: gmmktime() internaly uses mktime() so only times valid in derived local time can be used.