As Sara Golemon points out on her blog today, it's the 7th birthday of the very first release of PHP4:

Check the official releases page for yourself. PHP 4.0.0 was released seven years ago today. Make a cake, blow out some candles, and put on your birthday suit (no not that one); Now give that brat a kick out the door, he's been loafing around on your server for too long.

Of course, if you're still stuck in the PHP4 days, there's the latest - PHP 4.4.7 - or, if you're ready to join the rest of the world, PHP 5.2.2.