It's stats o'clock! See 2014, 2015, 2016.1, 2016.2 and 2017.1 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 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 then shares the results of the statistics including the total number of users for each version of the language and the difference in just the last 6-7 months. PHP 7.1 has pulled out as a front-runner with PHP 7.0 coming in closely after. He also shows some historical data showing the decline of the 5.x versions and the rise of 7.x versions over the past years. The post ends with information about the percentage of requirements in packages with 5.6 taking the lead but not by much.