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.
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. The PHP community has decided that it is time to move forward, together. Effective February 5th, 2008, any new feature releases will have a minimum version requirement of at least PHP 5.2.0.