Regarding PHP 5.3 support, I put together a quick report in WikiApiary showing the versions of PHP in use across wikis.
https://wikiapiary.com/wiki/PHP_Versions
In short, 5.3 is the most common PHP version used by a large, large majority.
Three things to footnote in this data (and you could run additional queries to get the real data for these).
1. WMF itself runs PHP 5.3, so that explodes the user count a lot. If you excluded WMF (based on an assumption that WMF controls it so thus could move to newer version easily) it lowers the active users on 5.3 dramatically.
2. A large percentage of the 5.3 install base is there because that is what Ubuntu is distributing (my farm is on 5.3 for this reason). If there was an easy PPA solution to move from 5.3 to 5.4 for Ubuntu 12.04 that would also lessen the dependency on 5.3.
3. If you queried by Netblock you could identify how many of these 5.3 users are on Bluehost, Dreamhost or other system where they have no ability to upgrade, the hosted would have to do that.
All data (except time series edit data) for WikiApiary is stored as semantic properties, so all of these things are available for #ask queries.