In this new post to her blog Lorna Mitchell takes a look at some of the current statistics around PHP version adoption - all the way from the ancient 5.0 through the shiny new (upcoming) 5.5 releases.
PHP runs over 75% of all websites whose technologies are known (source: w3techs), which makes for a really REALLY long tail of users who once installed wordpress, phpmyadmin, or some other open source project that helped their business needs at the time. What they don't do is upgrade. PHP's current usage statistics look like this (source and raw numbers are if you want them):
She points out that around half of the results show that sites are running on unsupported versions of PHP (<=5.2) but notes that it's not always their choice. There's lots of factors that play into upgrading these versions that are not always in the user's control (like the speed of distro updates). She covers some of the things that came around in the newer versions of PHP 5.2 and 5.3 including some large performance jumps, especially in 5.4.
In truth, the future is already here for those people on PHP 5.4 and beyond. Keeping PHP upgraded is just part of our regular maintenance workflow, and the language is progressing in regular and manageable steps. If you've been left behind then I strongly recommend that you start making plans for upgrading your platform, or moving to a newer one.