Netcraft.com has posted the results of a web server survey with data compiled starting in 2002 all the way up to 2012 about the growth and usage of PHP on the web. The title of the article, "PHP just grows & grows", gives a clue to their findings.
Netcraft began its Web Server Survey in 1995 and has tracked the deployment of a wide range of scripting technologies across the web since 2001. One such technology is PHP, which Netcraft presently finds on well over 200 million websites.
For those not familiar with the language, they give an overview of its history starting back with PHP v1 that Rasmus Lerdorf developed for his own uses. They move quickly through the years talking about versions and improvements made during their lifecycle. They also talk some about their own tracking methods and the metrics they use to measure PHP's growth - hostnames serving up PHP-based sites, removal of active (not spam) sites, unique IPs and actual computers/machines.