PHP 4 has served the web developer community for seven years now, and served it well. However, it also shows its age. Most of PHP 4's shortcomings have been addressed by PHP 5, released three years ago, but the transition from PHP 4 to PHP 5 has been slow for a number of reasons.
PHP developers cannot leverage PHP 5's full potential without dropping support for PHP 4, but PHP 4 is still installed on a majority of shared web hosts and users would then be forced to switch to a different application. Web hosts cannot upgrade their servers to PHP 5 without making it impossible for their users to run PHP 4-targeted web apps, and have no incentive to go to the effort of testing and deploying PHP 5 while most web apps are still compatible with PHP 4 and the PHP development team still provides maintenance support for PHP 4. The PHP development team, of course, can't drop maintenance support for PHP 4 while most web hosts still run PHP 4.
It is a dangerous cycle, and one that needs to be broken. The PHP developer community has decided that it is indeed now time to move forward, together. Therefore, many popular software projects like Drupal, phpMyAdmin, phpPgAdmin, Propel, Symfony, Typo3 etc. have all agreed that effective February 5th, 2008, any new feature releases will have a minimum version requirement of at least PHP 5.2.0. Furthermore, the listed web hosts have agreed that effective February 5th, 2008, they will include PHP 5.2 (or a more recent version) in their service offer.
It is our belief that this will provide web hosts reason to upgrade and the PHP development team the ability to retire PHP 4 and focus efforts on PHP 5 and the forthcoming PHP 6, all without penalizing any existing project for being "first out of the gate".