Usage Stats for January 2013
PHP: 244M sites, 2.1M IP addresses
w3techs.com provides statistics for
server-side programming languages
where you can also see the market share
See their FAQ and technologies page for the details of the methodologies used for the surveys.
You can also see how popular PHP is relative to other Apache modules at SecuritySpace's Web Survey.
There is also a Programming Community Index provided by TIOBE.