Usage Stats for April 2007

PHP: 20,917,850 domains, 1,224,183 IP addresses
Source: Netcraft

You can also see how popular PHP is relative to other Apache modules
at SecuritySpace's
Web Survey. Spoiler: PHP is the most popular.

There is also a
Programming Community Index
provided by TIOBE.

There are
detailed stats about PHP, by version, by countries, by web servers
and evolution from the "PHP version tracker", run by
nexen.net.