This is not a bugfix ;)
What about making SymfonyRuntime's constructor accept array|json-string instead?

This is not a bugfix ;)

Are you saying APP_RUNTIME_OPTIONS is not a supported env var? but more like some internal global var?

IMHO it looks like an env var extension point ... thus should support string values actually. Thus a bugfix :)

Or how could i set an PHP array using env var?

We're not at 6.1 yet, so as a feature i will postpone this PR. And we patch the front controller manually for the time being (maybe forever), if that's the intended feature-usage currently.

So, tbe bugfix would be

$ APP_RUNTIME_OPTIONS={} bin/console 
PHP Warning:  A non-numeric value encountered in vendor/autoload_runtime.php on line 23
PHP Fatal error:  Uncaught Error: Unsupported operand types in vendor/autoload_runtime.php:23

more like some internal global var?

yes, that's been the intent so far, not reading from env.

then, what about renaming it to SF_RUNTIME_OPTIONS for 6.1?

I don't see why, better not to me 🤷

Can you add a test case?

I'll continue this with our php8/sf6 upgrade 👍