user controlled config merging

gistfile1.txt

###
# uses http://yaml.org/type/merge.html
# imports read the given file and define implicit merge key alias for all possible paths
###

# config.yml
foo: bar

# implicitly defines a &ding merge key alias
ding:
    # implicitly defines a &ding.lala merge key alias
    lala:
        - shala
        - raba

app:
    charset:       UTF-8
    error_handler: null
    csrf_protection:
        enabled: true
        secret: xxxxxxxxxx
    router:        { resource: "%kernel.root_dir%/config/routing.yml" }
    validation:    { enabled: true, annotations: true }
    templating:    { engines: ['twig', 'php'] } #assets_version: SomeVersionScheme
    session:
        default_locale: en
        lifetime:       3600
        auto_start:     true

# config_dev.yml
imports:
    - { resource: config.yml }

# implicitly defines a &ding merge key alias
ding:
    # implicitly defines a &ding.pong merge key alias
    pong:
        - rara

# config_test.yml
imports:
    - { resource: config_dev.yml }

ding:
    pong:
        <<: *ding.pong
        - roro

app:
    <<: *app
    error_handler: false
    test: ~

# result
foo: bar

ding:
    pong:
        - rara
        - roro

app:
    error_handler: false
    test: ~
    charset:       UTF-8
    csrf_protection:
        enabled: true
        secret: xxxxxxxxxx
    router:        { resource: "%kernel.root_dir%/config/routing.yml" }
    validation:    { enabled: true, annotations: true }
    templating:    { engines: ['twig', 'php'] } #assets_version: SomeVersionScheme
    session:
        default_locale: en
        lifetime:       3600
        auto_start:     true