If you're a PHP developer and you don't know who Damien Seguy is, stop what you're doing and check out his work. Every month his scripts gather information from the PHP headers of tens of thousands of websites and server and then aggregates this information into something useful for the rest of us.

From these headers, he's able to determine and track the various versions of PHP over time and give the rest of the world insight into what's really going on. I'm not going to go into all the data in great detail - he already does that excellently - but I will cite this one:
This is one of the most interesting and to every mISV or Open Source Project, the most important. According to this, PHP5 penetration is only 17% or approximately 1 in 6 sites. As we're developing exciting new applications and tools, we need to keep this in mind. We need to be aware that some of our potential users and customers are still running on significantly different versions of code that have much smaller/different feature sets. These aren't necessarily old versions - PHP4 is still under active development - but they don't have some of the "modern" functionality that we've learned to love in PHP5.

And this is only going to get more exciting as we approach the release of PHP6. As soon as a target date is announced, any admin considering the PHP4 to PHP5 jump is going to put it off and we'll be right back where we started...

And if you're interested in deeper statistics, check out Damien's analysis of various public phpinfo() pages and the information he gleaned from them. Quite interesting to see the individual modules and configurations in use...

Hat tip: Cal Evans, Editor of Zend's DevZone

Disclaimer: I had the opportunity to meet Damien last year at the DCPHP Conference and ran into him again last week at the MySQL Conference. I happen to like the guy and sincerely appreciate his work.