Like Perl, PHP is about as Year 2000 compliant as your pencil. It is the applications you write with PHP you need to worry about, not PHP itself.

There is an issue with the dates in cookies. Netscape originally specified that the expiry date on a cookie should be in a 2-digit year format. Due to all the y2k hype, they decided to change this behaviour in Netscape 4 and up. This doesn't mean that the 2-digit year is not y2k compliant. A 2-digit year of "13", for example will be understood as the year 2013 in Netscape. All browsers understand this 2-digit format, and thus this is the default in PHP. Some y2k fanatics still insist on never using a 2-digit year no matter what, and for those people PHP has a y2k_compliance configuration setting available in the php3.ini file.