(To be clear, the Server header can already be set using Caddy config -- but this PR makes it possible to customize its static value which is optimized for the hot path.)

Can we make it a const to avoid a plugin/module manipulating it at runtime?

I don't think so (and it's convenient for FrankenPHP btw 😅)

With an exported var technically there's a data race

It's possible to make the variable unexported, but it will be less convenient for FrankenPHP (and maybe Framer btw).

The build flag still works ever on const though. How is a const inconvenient?

@francislavoie This is debatable. It should only called in an init() function and will be race free.

@mohammed90 so in FrankenPHP we can just change this variable instead of customizing our (plenty) build systems.

Why's there a data race by being exported?

Cause if someone writes a stupid plugin that runs during HTTP requests that tries to change the variable, it can cause race.

Oh. I see... yes, that's true (as with any exported package var, I guess)