Returns the Unix timestamp corresponding to the arguments given.
Values 1 to 12 reference the normal calendar months of the year in question.Values less than 1 (including negative values) reference the months in the previous year in reverse order, so 0 is December, -1 is November, etc.Values greater than 12 reference the appropriate month in the following year(s).The number of the day relative to the end of the previous month.Values 1 to 28, 29, 30 or 31 (depending upon the month) reference the normal days in the relevant month.
Values less than 1 (including negative values) reference the days in the previous month, so 0 is the last day of the previous month, -1 is the day before that, etc.
Values greater than the number of days in the relevant month reference the appropriate day in the following month(s).
The number of the year, may be a two or four digit value, with values between 0-69 mapping to 2000-2069 and 70-100 to 1970-2000.
On systems where time_t is a 32bit signed integer, as most common today, the valid range for This parameter can be set to 1 if the time is during daylight savings time (DST), 0 if it is not, or -1 (the default) if it is unknown whether the time is within daylight savings time or not.
If it's unknown, PHP tries to figure it out itself.
This can cause unexpected (but not incorrect) results.