But I wouldn’t only limit this to Open Source projects. Even in companies working with PHP, Frameworks, Libraries, etc. you will face this problems, too. As an example you will have customers that don’t want to upgrade their software just because it runs on a new PHP version.

Because of all this we are working on a plan atm to run multiple PHP version to make the switch as smooth as possible at the company I’m working for.

If a new project is starting up they have the ability to say “I’m only supporting PHP 5.3″ they won’t catch much guff. That project can dictate what they want to do and yes, they might loose some users because shared hosts won’t upgrade, but the overall net gain as PHP 5.3 increases in market share means less support up front and possibly a better position down the road.

The other types of projects, like Zend Framework, normally will have an established maintenance plan in place. New whiz-bang features will slowly move from being in the older 1.x branch to 2.x, but that doesn’t mean development won’t completely stop on 1.x. Security patches and general bug fixes will still happen until that branch can formally put out to pasture as per the maintenance plan.

The whole version argument really hits home with enterprises. Look at all the people running Redhat that are still on 5.1.x all because their managers don’t want to leave the supported versions RHEL has established. In those cases though, they probably aren’t running bleeding edge software anyway so again, not a huge impact. The more agile enterprises will already be planning their 5.3 migration (I know I am) and won’t have a problem with software that is 5.3 only.

(There is also the fact that most well-written 5.2 code will run fine without any modifications will run in 5.3, so its more of a political and business issue than a technical one in my mind).

Related to your first point about WordPress, is WordPress in the same boat as Microsoft was with IE6? Are we going to see a 6-year drought between any real progress or an increase in minimum recommendations just because their user base is “too big to fail?” Not making a necessary change because it would affect too many people is like not ending a marriage because it would hurt too many other people.

This “getting away with” requiring PHP 5.3 is hot air. If anything, the next version of WordPress SHOULD require a modern PHP version so it can lend itself to modern best practices and enhanced security. Being such a widely used application, that alone should be paramount. If the core WordPress developers started on a new version of WordPress requiring PHP 5.3, by the time it’s done, 5.3 might have more adoption. This is exactly the mindset of major framework developers for symfony and Zend Framework.