php7.1 isn't in the
stretch repo, but it is in the
sid repositories (for Debian 10 and unstable, respectively).
You should be able to just change
buster, and then run:
$ [sudo] apt-get install php7.1
You may find it helpful to upgrade your system as recommended in the Debian documentation when doing this using
apt-get update && apt-get upgrade — be aware that this will upgrade your entire system from stable to testing, and many packages are likely to be upgraded.
On to your bonus question:
Where should I drop the request to update the default PHP version of Raspbian?
The default version of PHP in Raspbian is extremely unlikely to be changed for now, because of the way that Debian's release cycles work:
In the above array, data closely following "current" releasing tendencies have been highlighted in green. What can be deduced from those data is that the "most-typical" Debian release:
- endures a freeze cycle of 7 +/- 1 months before getting released.
- is released about 2 years after the previous one (the often cited example of Debian Sarge being quite an exceptional event in Debian history).
- leaves users about 1 year to upgrade to the next one once this latter itself gets released.
- has (from release to the end of security updates) a total lifetime of about 3 years.
Stretch was frozen in February 2017, so there will be no new feature updates to the PHP package in the Stretch repositories. So, for the foreseeable future, the PHP package will remain outdated in stable until Raspbian Buster becomes stable (at which point, the PHP package there will become outdated, ad infinitum).
It's just the nature of Debian: you trade away the newest packages in exchange for greater stability, and you'll have to stick to the testing release if you want the latest features.