Conditional Runtime PHP Extension loading

Description

I would like to build a docker container with various PHP extensions and be able to toggle them off with environment variables. (OpenTelemetry / XDebug / Tideways / Blackfire does not like each other). Of course I can build various tags, but it's an hassle of CI managment.

The solution who most people are doing is to use the docker entrypoint to move files to disable loading of extensions. This does not work when the container is read-only. Therefore I would suggest to have some kind of environment variable to disable / enable extension on the fly.

Several ideas I had so far:

in php.ini

{if hasEnv("FOO")}
extension=foo.so
{/if}

so we can toggle even php.ini settings

or a simple

PHP_EXTENSIONS_DISABLE=opentelemetry

The issue does not name implementation files, tests, or entry points. Start by tracing PHP's extension-loading and configuration handling, then determine how environment-based enabling or disabling should work, including the proposed php.ini conditionals. Done means an agreed design is implemented and covered for read-only containers and runtime extension toggling.