Environment variables are becoming increasingly popular to manage the application configuration. They are one of the main concepts of the twelve-factor app methodology. Their main advantages are that they can can be changed between deploys without changing any code and that they don't need to be checked into the code repository.
Symfony has supported environment variables since day one thanks to the special SYMFONY__ variables. However their behavior was rather simple: when the application container was built, the values of those environment variables were dumped into the compiled container. Therefore, if those variables changed during the application execution, the updated values were ignored.
In Symfony 3.2 we added full support for environment variables. The first
improvement is that you no longer have to prefix those variables with
(any variable name will work). The second improvement is that their value is
resolved at runtime, so the application will always use the updated value.
In order to use an environment variable in any Symfony configuration file, use
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# app/config/config.yml doctrine: dbal: # ... password: "%env(DB_PASSWORD)%"
In the previous example, whenever Symfony/Doctrine need the database password,
they'll make a call to get the value of the
DB_PASSWORD environment variable.
To avoid undefined variable errors, you can define default values for these
variables using the syntax
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# app/config/parameters.yml parameters: env(DB_PASSWORD): s3cr3t_1234
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