Each zone is identified by the city with the largest population in a specific area.

Zones are divided into 10 major groups: Africa, America, Antarctica, Artic, Asia, Atlantic, Europe, Indian, Pacific.

Examples are: America/Toronto, Europe/Berlin, America/New_York, Europe/Oslo, Asia/Singapore.

There is also a group "Others" which contains outdated and backwards compatible names, such as Canada/Eastern, Etc/GMT-1 and US/Central. Those should not be used.

Pick the zone that is closest to your location.

http://php.net/timezones