Identical to mktime() except the passed parameters represents a GMT date. gmmktime() internally uses mktime() so only times valid in derived local time can be used.
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.
The hour
The minute
The second
The month
The day
The year
Parameters always represent a GMT date so is_dst doesn't influence the result.