What should the minimum supported version be?
There are two considerations:
- Soonish, Parsoid will be set up as a PHP service in the Wikimedia cluster, so obviously it needs to support whatever version of PHP we intend to use there.
- Maybe less soonish, Parsoid will become a part of the MediaWiki distribution (at the latest after the parser unification, but possibly even in its semi-standalone form), so it should probably support all PHP versions that MediaWiki supports.
Current WMF migration plan is T176370: Migrate to PHP 7 in WMF production - HHVM to PHP 7.2 by 2018-19 Q2 (I guess the timeline in the task description is outdated?). The task for dropping third-party support for HHVM is T176370: Migrate to PHP 7 in WMF production which is unclear about whether that's supposed to happen immediately after the WMF switch of later. I don't think there's anything in Phabricator about plans for dropping 7.x support for some x.
Per the PHP support schedule, 7.0 is end-of-life with no security support; 7.1 is end-of-life and security support will be dropped next December.