Description:
------------
http://php.net/mktime

"mktime() returns the Unix timestamp of the arguments given. If the 
arguments are invalid, the function returns FALSE (before PHP 5.1 it 
returned -1)."

but it returns "Tue Nov 30 00:00:00 1999 +0200" instead when invalid 
input like null (or undefined) is used:

php -r 'echo mktime(0, 0, 0, null, null, null), "\n";'
943912800

ok, i understand if all params are 0, then it would make sense:

"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 year is somewhere between 1901 and 2038. 
However, before PHP 5.1.0 this range was limited from 1970 to 2038 
on some systems (e.g. Windows)."

Year as 0 is 2000,
Month 0 is calculated as 12 of the last year, thus it gets December 
and 0 December is 30 November

please make using undefined variables and nulls being invalid input 
(so that you must cast to int, to treat empty input as 0):

php -r 'echo mktime(0, 0, 0, (int )$_GET['day'], 
(int )$_GET['month'], (int )$_GET['year']), "\n";'