The output from symfony help new gives:
Description:
Create a new Symfony project
Usage:
symfony local:new [options] [--] [<directory>]
Arguments:
directory Directory of the project to create
Options:
--dir=value Project directory
--version=value The version of the Symfony skeleton (a version or one of "lts", "stable", "next", or "previous")
--full Use github.com/symfony/website-skeleton (deprecated, use --webapp instead)
--demo Use github.com/symfony/demo
--webapp Add the webapp pack to get a fully configured web project
--api Add the api pack to get a fully configured api project
--book Clone the Symfony: The Fast Track book project
--docker Enable Docker support
--no-git Do not initialize Git
--upsun Initialize Upsun configuration
--cloud Initialize Platform.sh configuration
--debug Display commands output
--php=value PHP version to use
What does the --php=value parameter do?
"PHP version to use" means what, exactly? Use where, and when?
Are these options documented in more detail online somewhere?
Basically, the only thing this option does is to create the file .php-version and populate it with your chosen value.
That value will subsequently be used by the Symfony CLI to choose a different PHP runtime to execute commands, as described here.
This PHP version will be used when running commands through the Symfony CLI wrapper (e.g. symfony console foo:bar), or even (as far as I can tell), as a runtime when using the Symfony CLI built-in server.