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.
Parameters
hour
The hour
minute
The minute
second
The second
month
The month
day
The day
year
The year
is_dst
Parameters always represent a GMT date so is_dst
doesn't influence the result.