Nexen.net points to their latest PHP statistics (in French) for the end of last year - December 2005.
Here's the latest statistics on the usage of PHP for December 2005:
Also this month, you can see some of the graphs representing other various stats since June 2005.
Some of the other items they measure include: adoption by country, number of servers running it by country, and the adoption of PHP in Canada for the month. Note: the page is all in French, but the graphs are easy to understand...