Every PHP programmer wants their application to run on as many servers and web applications as possible. In order to do this, you need to write for PHP 4. Unfortunately, PHP 4 has been discontinued, PHP 5 has been here for 3 years and PHP 6 is right around the corner.
There weren’t very many web hosts supporting PHP 5 a few months ago. Things have been changing due to the effort of many PHP 5 evangelists. The new PHP is a better, more secure language. The many security issues of PHP 4 will not be available in PHP 6. Version 5 is meant to be a transition to convert code for the upcoming major release.
I’ve written WordPressXmlRpc for PHP 5. When I released it, there were a few people who wanted it run on PHP 4 so I converted it. The application is relatively simple so it wasn’t much of an effort. I’ve also written a PHP application for backing up databases and folders on my web hosting account. This application will be released in the coming weeks. Its written in PHP 5 and is very object oriented. Converting it to PHP 4 will be quite a task and not one I’m interested in doing. There are so many web hosts that support PHP 5 and it takes too many resources maintaining two versions of the same software.
All PHP applications should be written for version 5 and there is still plenty of time to convert older applications before version 6 is released.