As far as i know, PHP used UTC timezone to show the date..
And to see the timezone that your PHP version used you can open phpinfo, then you can refer to "Default timezone"
the default timezone is UTC.. UTC is Coordinated Universal Time, GMT is Greenwich Mean Time... both are the same..
example : Los Angeles, California, United States: UTC−8
Sydney, Australia: UTC+10
to refer more about GMT you can visit : http://en.wikipedia.org/wiki/Timezone