Damien Seguy just sent me links to Nexen’s PHP Adoption Statistics for March 2007 (also an analysis of the PHP adoption statistics for March 2007).
Buried in the data is one little gem: if you want to run on the majority of public-facing PHP installations, target PHP 4.3.10 or later. That represents 60% of the PHP-enabled hosts.
There are other interesting statistics, too. Apparently PHP 5.2 is taking over from PHP 5.0 and 5.1, while PHP 4.4 is still dominant. Even more interesting is that Apache httpd 1.x is almost three times as popular as Apache 2.x.
I’ve wondered before if people upgrade. So far, few do.