Every six months Jordi Boggiano, co-creator and maintainer of Composer/Packagist, publishes statistics on which versions of PHP are used. Some good news: PHP 7.1 is the most used version.
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.