Add support for FrankenPHP

[jwage]

I’m interested in using FrankenPHP with Heroku. Would it be possible for me to add support to this buildpack or would a separate buildpack make sense?

https://frankenphp.dev/

[jwage]

@dzuelke thoughts on this? Would you accept a PR?

[dzuelke]

Hi Jon,

I'll need to look into it, it's been on my list for a while now.

The main question is whether it can be done as an alternative PHP runtime that's somehow selected for installation, or if it needs to be a separate buildpack.

Composer has no notion of FrankenPHP as a runtime, but since it needs ZTS anyway, maybe if we start providing ZTS builds, we could always include it there.

The other alternative is for the buildpacks' ZTS builds to enable the embed SAPI, and then have a separate, lightweight FrankenPHP buildpack on top, that uses the shared library.

I've created an issue (#695) to track the ZTS efforts more generally. Please note that that work is likely to not happen for this buildpack, but instead only for the CNB over at https://github.com/heroku/buildpacks-php.