Introduce a PHP Start buildpack about php

[sophiewigmore]

@dmikusa-pivotal I kind of think that the PHP HTTPD and PHP Nginx buildpacks should be fully rid of any and all start commands. In my eyes the PHP Start buildpack should have:

• HTTPD + PHPFPM + procmgr
• HTTPD + PHPFPM + procmgr
• HTTP only but as a proxy to PHPFPM external
• Nginx only but as a proxy to PHPFPM external

I agree that the PHP FPM buildpack should still handle the PHP FPM only case.

[dmikusa]

I like this approach.

The following scenarios come up to me:

What goes in the container	What buildpack sets the start command
HTTPD + PHPFPM + procmgr	PHP Start
Nginx + PHPFPM + procmgr	PHP Start
HTTP only but as a proxy to PHPFPM external	PHP HTTPD
Nginx only but as a proxy to PHPFPM external	PHP Nginx
PHPFPM only	PHP FPM

I think the existing buildpacks can cleanly handle the last three cases, but it'd be tricky for them to handle the first two. Adding a PHP Start buildpack to handle those cases will keep PHP FPM, PHP HTTPD, and PHP Nginx simpler.

The only downside I see here is that things are getting quite spread out. There are now four buildpacks that can set a start command. You have to have a good understanding of how that works and who's responsible for those scenarios to a.) make future changes confidently and b.) troubleshoot if something unexpected happens.

I definitely think this requires some specific dev docs and possibly a flow chart or something to document how this is expected to behave.

[sophiewigmore]

@paketo-buildpacks/php-maintainers if we're in agreement I will close this out and write the RFC!

[sophiewigmore]

RFC has been merged so closing this out!