We came across this one recently when setting up a Zabbix server, where the installation would not proceed until the PHP timezone was set. This is quite straightforward – using your favourite editor edit:
/etc/php5/apache2/php.ini
Look for the Module Settings section or search for:
date.timezone
(in vi /date.timezone should get you there). Uncomment the line by removing the leading ; and then add in your desired timezone. For Sydney it would look like:
[Date]
; Defines the default timezone used by the date functions
; http://php.net/date.timezone
date.timezone = UTC+10
Change UTC+10 to your desired timezone, save the file and restart Apache:
sudo service apache2 restart
Voila!