Right, that "Y2K7" is not a typo. It "refers to the Year 2007 problem, caused by a US-mandated change to Daylight Saving Time" ( wikipedia ). From this year the US (and Canada) changed the dates on when daylight saving time is active. DST now starts three weeks earlier, and ends one week later. This causes quite a lot of problems with software that hard coded those rules. On Linux (and other Unix-like operating systems) this can be remedied by updating the zoneinfo information - check your Operating System's documentation on how to do that.
PHP before PHP 5.1.0 relies on the Operating System to provide time zone information, while PHP versions 5.1.0 and higher come with a bundled time zone database. The database that ships with PHP 5.1.0 already covers the changes in the US and Canada, however, the US and Canada are not the only places changing rules. Many places on earth change those rules all the time which means PHP's database needs to be updated as well.
Every PHP version comes with the latest release of the time zone database at the moment PHP is released. However, it is also possible to just update this data by installing the PECL extension timezonedb . This extension is released whenever a new version of the Olson database becomes available. The current version is 2007c (2007.3).