Jordi Boggiano, the creator of the Composer project, released PHP version stats for May of 2016 and a comparison against last November.
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.
The results are interesting and it shows a big swing in people adopting PHP 7.0:
A few observations: 5.3 dropped to almost nothing which is great news! 5.4 is also down by almost 10% and is definitely on the way out. 5.5 is still big but less so, while 5.6 got a huge boost to become the main version. The big surprise is that we have 20% of PHP7 already! That is great news only six months after this major release came out.
It is great news to see PHP 7.0 gaining so much, but as Jordi said, this is just a subset of the PHP ecosystem. I have to imagine actual adoption is much lower.