Ok no, its not the Y2K bug again, though it is definately the php programmers version of it. The transition to php5, are your websites and scripts ready? I was recently working on an open source project transitioning it form php4 to php5. It was hell, a TON of work and a lot of changes. So if you are a php programmer start getting into php 5 because soon most hosting providers will switch to it and your scripts will need to adapt. If anyone is interested in me helping them with this for thier websites or scripts feel free to pm me.
My experience has been the opposite. Scripts I have written work unchanged under PHP 4 and 5. In fact, all of the third party scripts I use work without a problem under the latest version of PHP. If one is concerned, install PHP 5 on a test machine and test scripts there. The real trick in programming is to ignore undocumented features and to make changes in syntax required by languages before they are mandated. This will not prevent problems, but it will help minimize them. Think standards and strict programming practice at all times.