So this is just a little tip when you need to load your own yaml file.
To load a yaml file you can use sfYamlConfigHandler::parseYaml() ( or sfYaml::load() ). And just after you can also use sfYamlConfigHandler::replaceConstants() to replace constant(s) from Symfony such as %SF_LIB_DIR% (or any other value defined in a yaml config file) in the values you've just loaded from your custom yaml file.

A little exemple: