this method relies on your operating system, to make it so that it always works, just store the excisting timezones in an array/database, also you need to take into account the daylightsavingstime(s) wich can be quite some work
sourece http://en.wikipedia.org/wiki/Time_zone:PHP
The DateTime objects and related functions have been compiled into the PHP core since 5.2. This includes the ability to get and set the default script timezone, and DateTime is aware of its own timezone internally. PHP.net provides extensive documentation on this.[20] As noted there, the most current timezone database can be implemented via the PECL timezonedb.
so either make the db yourself, or use the PECL timezonedb