A Symfony Config component allows to create and load a configuration. For instance, it allows to use a yml for config.
However, in a Symfony framework configuration uses parameters with a special syntax -
So I'm wondering what's magic used to replace placeholders in a yaml config file?
The only goal of the parameters is to be stored in a container parameter bag.
Not quite. Special compiler pass resolves the placeholders. It works at the optimization step of the container compilation and inject actual values, so that placeholders can be used in the service definitions and in the semantic configuration files.
... and I've got an error since aBooleanKey requires a boolean value but got a string.
I guess that caused by YAML Component:
Finally, there are other cases when the strings must be quoted, no matter if you're using single or double quotes:
- When the string is true or false (otherwise, it would be treated as a boolean value);
So, in your example
true is quoted and treated as string.