It's stats o'clock! See 2014, 2015, 2016.1 and 2016.2 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 month 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 starts with the differences between now and the last time he ran the stats with a nice trends towards the PHP 7.x releases, especially PHP 7.1. He shares some graphs of the overall version distribution and a time-related graph showing changes in usage over time. Finally, he ends the post the same way as the others showing requirements of packages and how they've changed since the last update (what version a package requires).