In his latest post Jordi Boggiano (of the Composer project) has released his PHP usage statistics for the second half of 2016 based on the information gathered during Composer installations.
It's stats o'clock! See 2014, 2015 and 2016.1 for previous similar posts.
A quick note on methodology, because all these stats are imperfect as they just sample some subset of the PHP user base. I look in the packagist.org logs of the last 28 days for Composer installs done by someone. Composer sends the PHP version it is running with in its User-Agent header, so I can use that to see which PHP versions people are using Composer with.
He compares them to the statistics from May 2016 showing some interesting but not unexpected changes, mostly in the growth of PHP 7+ versions. He shares a few of his own observations of the results and encourages library authors to start focusing on PHP 7 functionality rather than 5.5/5.6 compatibility. He also shares a secondary data set - the PHP versions that libraries require that, surprisingly, is moving a lot slower than the actual PHP version adoption.