At long last, the WordPress project will be ending support for PHP 4. WordPress 3.1, to be released in late this year, will be the last version to support the legacy version of PHP.
For WordPress 3.2, due in the first half of 2011, we will be raising the minimum required PHP version to 5.2. Why 5.2? Because that’s what the vast majority of WordPress users are using, and it offers substantial improvements over earlier PHP 5 releases. It is also the minimum PHP version that the Drupal and Joomla projects will be supporting in their next versions, both due out this year.
The PHP developers themselves dropped support for PHP 4 back in 2008 in order to focus on the PHP 5 branch and beyond. PHP 5 was released six whole years ago and has many improvements over PHP 4.
Mark Jaquith in his announcement says that only 11% of WordPress installs are running versions of PHP under 5.2, and that the decision will likely give those few hosting providers the necessary kick.