..tip::

..versionadded::3.3

The support of the special ``SYMFONY__`` environment variables was

deprecated in Symfony 3.3 and it will be removed in 4.0. Instead of

using those variables, define regular environment variables and get

their values using the ``%env(...)%`` syntax in your config files.

You can also define the default value of any existing parameters using

special environment variables named after their corresponding parameter

prefixed with ``SYMFONY__`` after replacing dots by double underscores