Usage Stats for April 2007

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

Graph

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.