I don't recall seeing anything in EE core code that sent this data, but some of the larger add-on developers include stat tracking within their code that reports version numbers, etc. Possibly they may be willing to share the information, but that will only be a small selection and example of users who use that add-on, not EE in general.
The only reliable way would be if it was built into EE and sent anonymous stats back to Ellis Labs - but I doubt they'll want to share that data with the community.
So the only reliable way would be to include tracking in your add-on that regularly (e.g. once a month) when accessed (either frontend or control panel) triggered a cURL that posted data data to your website carrying that information (PHP version & EE version). As long as it was clearly stated in the T&Cs or documentation, or you offer it as turnable off in the settings (but then you won't be accurately tracking all your user base).