I've seen a few reports on the adoption rate of PHP 5. There's another one out today. All seem to conclude that PHP 5 adoption is at about 5%.

This relates to one of the best decisions I made regarding HelpSpot. About a year ago when I was doing the requirements I had a big decision to make. Support PHP4 + 5 or only 5. I was very very close to supporting only 5 because it would allow me to make things more OO (not that big a deal to me) and more importantly make use of newer functions like simpleXML and so on.

As it turns out supporting both 4 and 5 was definitely the right way to go. With adoption of 5 still at miniscule levels it would have really put HelpSpot in a bad position. Word on the street is that the PHP guru's are jumping right from 5.1 to 6 so it could turn out that PHP 5 will be the version nobody ever installs and they all jump right to 6 when it's released.

So if you're doing or going to do a PHP app you better keep PHP 4 support at least for the next year or so.