According to this new post on Andi Gutmans' blog today, the adoption of PHP versions seems to be tipping in favor of PHP5.
PHP 5 was released in July 2004. Since its release two years ago, there has been a lot of talk about its adoption rate and whether it will surpass the PHP 4 installed base. No doubt that it will still take some time for PHP 5 to reach PHP 4's existing installed base. The main reason for this are the numerous PHP 4-based applications that were built over the years, and if it's not broken, don't fix it.
He includes a graph showing the adoption of both PHP4 and PHP5 versions, noting a large increase from the release of the 5.1 version of PHP.
Although I knew many were waiting for PHP 5.1 before upgrading, its release in last November seems to have created a huge increase in PHP 5 adoption. In July over 74% of support tickets were by customers using PHP 5 (and remember we do support both versions).