Here's the latest statistics on the usage of PHP for December 2005:

• PHP 5.x usage grows up to 5% (at the same pace as months before). The release of PHP 5.1 seemed to have no effect.
• PHP 4.4.4 continues to enjoy strong use, despite the differences between it and older versions like 4.3.10 and 4.3.11.

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...