There is a very nice phpbb demo site:
choose Admin Page and login with given credentials (link at the bottom of the page). Then the last position in the menu is ‘Search settings’. If you try to do this, you will see what I mean by ‘add phpbb support for manticore’ - I guess it should be possible to make a pull request for a new search option.
I am not phpbb developer, but I think, that sphinx config (which you can generate on the above admin page) was written in 2012 for Sphinx 2.0.1.
For this reason I suppose that this config may be outdated. For example, specifying custom location of the pid file seems to be not compatible with systemctl start/stop (at least this is the case in my installation - executing searchd directly still works, but it leads to other permissions problems). My idea was, that it would be very nice to have a modern config for sphinx/manticore, and the addition of the support for manticore can be a good opportunity for this.